I am in a similar- although not the same, situation. We started working on 'leaving blankie in bed' when dd was around 3.5 - and she did really well for the most part. Then, all of the sudden a few months ago, she started wanting it ALL the time. Asking to bring it places (and we have NEVER brought it places), whining for it when we've been out for awhile, etc. Anyway -Because I am sensing this is a regression due to fear of starting K in 2 weeks, I have allowed her to bring it a LOT more places than I usually would (we do leave it in the car though) but I have told her that when she turns 5 that is it and blankie stays in bed starting then, no exceptions. She turns 5 on 10/21, so that will give her almost 2 months to get used to school before we do that. However, dd does know that blankie cannot come to school with her, and she's ok with that - but I know she'll RUN to that blanket when she gets home.
I'm not so sure about the ideas from other posters to let her bring it in her backpack...Personally I think that would make MY situation worse because once I allow it to happen - it is something that is 'allowed' and she will scream and cry for it if/when we decide to no longer allow the blanket. Also, knowing my dd, she would be sneaking over to her backpack all day long at school to 'see' her blankie - even if she isn't supposed to be there and instead of interacting and playing with the kids - I think it would isolate her even more than her shyness is bound to. So anyway - consider carefully before you allow her to bring it even once. I'm in agreement with the telling her that she's a big girl now and blankie will wait at home for her.
And for anyone with younger kids and blankies - this is a huge tip to not let that blankie out of the house at a young age - don't let it go to preschool, etc. - then you won't have this issue!
Good luck!