The only advice I would have is to try to put everything in writing where you can. Write an email to the class teachers and the principal outlining what your concerns are and why. Ask the teachers for written reports on the children's progress (not the 'normal' sort of school reports but in depth individual reposrts for YOUR children). Ask if they are meeting the state averages in their developmental areas. Perhaps contact the board of education and see what you or they (the Board) could do?
Is there some way you could get them independently assessed?