Hi, what exactly does she mean by slow?
There is a lot of variability in 5 year olds and some come to school with a wide span of knowledge and others don't. She's comparing your child to the others in his peer group which may not be fair if this is his first school experience and not theirs. But I would try to listen to her and figure out exactly what she is telling you. I've found teachers to be pretty good at stacking up my child vs others and knowing where they should be developmentally (even when I can't as a biased parent!).
So, I'd find out more information before being upset. Good luck.
I agree with specialmom.
However, its also possible that he is a bit shy or not used to this new school. If this is his first school, that's a possibility. Whatever, listen to his teacher because she has many kids to compare him to and you have only one.
Ask the teacher what you can do to help him to catch up with other kids whether it is to do some work at home with him, more reading, any sort of therapies, etc. She may not have anything for you to do to help him but if you ask, you'll at least know. Maybe some extra tutoring outside of school will help. Good luck.