I don't have a suggestion on the bus, but for HM I do. We have a DD with Spina Bifida who rides a wheelchair throughout the parks but walks onto most rides. She has limited balance so we either hold her hand in crowds or avoid them. On HM if you go to the opposite side of the exit in the stretching room and allow all of the crowd to exit before you head out you avoid all or nearly all of the jostling.
That is a good suggestion. If you stay close to the place where you entered the room in the first place, you will be farther from where the line exits into the boarding area.
Another alternative, with a Guest Assistance Card or a mobility device is to skip the Stretching room altogether. Because my DD can't walk, we need to skip that room with her and go directly from the line to the accessible boarding spot. That avoids the stretching room entirely.
Check with the CM at the entrance.
you can get a handicap pass at guest services (good for only in the parks) that will let you bypass the standard waiting lines and let you board at the handicap spot.
you will need a official letter from your doctor to show to guest services. this info is available at disneys godisney.com
we have taken injured,perm. handicaped and autism children to wdw and it works out great. as for the bus only our wheelchair people were aloud to board first.
bye uncle duck
There is information about Guest Assistance Cards in post #6 of the disABILITIES FAQs thread.
The official Disney website does not have anything about Guest Assistance Cards on their pages about disabilities.
You don't need a doctor's letter (and, in fact, most CMs won't look at one).
It is not meant to bypass lines or shorten waits - that information is printed right on the card. Most attractions are Mainstreamed, so that everyone waits together in the same line.
Transortation must accomodate disabilities just like (actually there are in some case more stringent standards) any other public accomidation. The core question in your case is does your situationt rise to the level of a disability under ADA. If it does then yes you can request to preboard. It is really no different thatn the accomidations for flying (except that it is coverd by a different section of the code, but hte same standsrd apply). In the end it is about equal accomidation.
GAC is a tool devised to attractions so no it does not have the same value, but can be an indicator for the driver that you have declared that you have a disability but the stamp may have no meaning.
Bookwormde
The reason for preboarding guests with wheelchairs is not an accommodation; it is because it is easier and more efficient to board a bus with a wheelchair or
ECV before other guests are on. It is more for the bus driver and other guests (since it is faster) than for the person with a wheelchair or ECV. The lift is at the back of the bus and the driver must close the front door, walk to the back and operate the door and lift to put the wheelchair/ECV on.
In city transit systems, everyone waits in line together and the guest with a wheelchair boards when they get to the front. WDW does have lines set up for that, but it doesn't work well having guests with wheelchairs wait in line; by the time they get to the front, the bus is going to be to full to board. This is different than city buses, which often have their lift and accessible seats in the front of the bus.
Flying is covered by the Air Transport Act and has specific standards written into the Air Transport Act defining pre-boarding, who it applies to, etc.
This is above and beyond what is in the ADA.
There are no similar guidelines in the Federal Transit Administration rules for
Transportation Services for Individuals with Disabilities. Those deal with making things Mainstream Accessible, non denial of service for riders just because they have a disability, service dogs and making rules for the ParaTransit systems for individuals with disabilities who are not able to use the mainstream accessible buses/routes.
Even with a VERY visible reason for boarding first - a wheelchair - we have heard many comments over the years from people who do not see why we should be boarding before they do. And, many of the comments are not nice, even if the other guests have gotten a seat.
PREBOARDING guests without a very visible need would be an invitation to a riot at a busy place like WDW and would not be a reasonable accommodation for that reason. There have been enough really nasty threads on the DIS Boards over the years about who should get a seat, etc to show what sorts of problems would occur. And, there was a recent situation where a guest actually attacked a bus driver because he thought he should get priority on a full bus with a child.
There are seats at the front of the bus that have signs that say that people riding in those seats must give them up to guests with disabilities or the elderly. There are also signs over the seats in the back of the bus that fold up to make a wheelchair spot. The bus driver can ASK people to move, but even in the case of the wheelchair spots, if the person sitting there refuses to move, the driver can't make them. That is the extent of protection to sit on the bus.
As others suggested, if you want to make sure you get a seat, the best bet is to wait for the next bus if you see the one you would be getting on is too full. We have seen people get to the front and then step aside at the front to wait for the next bus because they want a seat. Other people who don't care if they stand just go around and board the bus.
When leaving the park, by waiting a few minutes after a parade or fireworks show, you will also have much less crowded conditions. You will be able to walk out without worrying about people hitting into you and when you get to the bus stop, the first few buses will have left. In our experience, the people waiting for the first few buses are the ones in the most hurry to get home and are usually the least patient.
Having a hidden disability is hard sometimes! 
, Maybe if they allowed GACs to be used for that then you would qualify, but since they don't allow GACS to be used outside of the park I am not sure that they can accommodate you. Maybe you can ask the driver and she what the results are? 
I needed help with my bags too or I would have just wanted because I had an "A" boarding pass.
