Sounds like the school is out of budget for more trips or the rates to hire a good, reputable bus company went up, after that horrific school bus crash in NJ, a couple weeks ago, where the whole top section of the school bus was completely ripped off the undercarriage after a dump truck couldn't stop in time when the school bus driver made an illegal U-turn, and the OP's school can't make up the difference in price, and is deciding to use parents as drivers instead. That school bus accident was one of the worst vehicular accidents I've ever seen, forget
school bus accidents.

It made national news and some of NJ cancelled bus field trips after. NJ is now in the process of changing certain laws. The bus driver had previously had his driver's license suspended 14 times.

The Board of Ed had even known about the current suspension and told the Superintendent NOT to hire him. But, over the school break, he got his license reinstated, so the Superintendent rehired him for the tragic trip.
So possibly,

this school has been impacted by news of this accident, cancelled funding current
bus trips for the time being, till they review their own procedures and vetting of bus companies. Yet the school is seeing if they can make the trip work with parents driving instead and stating if parents allow their kids to go, the school is not liable, just like if a parent had planned a trip on their own and allowed another parent to drive their kids? People get into cars all the time with other people driving where it hasn't been researched what is the current status of their driver's license or what their driving history is: for work carpools, school carpools, trips to sporting events, trips to the mall, to the movies, on dates, etc. Most trips end up fine. The school may be hoping for that. Also, since it's not an
educational trip but a fun event trip, it is totally on the parents if they allow their kid to go?