Why can't my son be brave and stand up for the lord?

My son is in seven grade and his sinece teacher give him some homework to do about how humans evolved. We are christens and I told him to tell his teacher that we didn't come from monkeys and that God made us end of story. He don't want to because he has a 97 in that cl and he is scared she will give him a f and make him not do to good for his grade card. I told him he has to stand up for God even if he fells that cl because our first obligaton is to him. He got mad and started crying and saying I am wrong but I know I am rite. I have prayed a lot and God told me that I am rite so I wish he would just do the rite thing and stand up for God and Jesus. I told him I will whip him if he don't do what I say and I don't want to do that but he has to learn that being a Christen means that sometime you have to do hard things for the lord. I guess he aint as good of a Christen as I thought he was but hopefully I can get through to him. Please pray for him.