Yes, we had to advocate like MAD with our Montessori preschool! In fact, the staff ignored my advocating entirely, and only skipped our son ahead to more "academic" work once my husband spoke to them. Ugh...
On the bright side, they did FINALLY listen, albeit grudgingly. In a day, he breezed through a typical week's worth of work: problem solved!
In my opinion, if suggesting that they try more challenging work for him as a potential solution to his alleged discipline issues doesn't get a favorable response--and I'd try the "Look, just try it with him for a day. What have you got to lose?" approach--then get him out of there. You can give him much of what he needs at home. For the rest, find/create a good playgroup, or even join a homeschooling group with lots of gifted kids in it. There will be social outings of families with gifted kids, and many of those will have little brothers and sisters tagging along.
Worst case, hang out at the local children's museum, and visit the park on nice days or an indoor play area on the bad weather days for his socialization. It's not ideal, but it's better than nothing...certainly better than a preschool that can only see him as the "bad kid." He deserves better!
Best wishes!
