MIL driving me nuts - How to cope?

You need to do what is best for you and your family. If your husband agrees with homeschooling and this is a decision you made together, then homeschool your children. The next time she complains to you simply say "I don't expect you to agree with the decision your son and I made together. I do expect that you will be respectful about it and any other decision your son and I make regarding our family." If she says that she cannot rescpect your decisions, simply say "I am sorry to hear that. I must go now" hang up the phone or leave. From that point on, distance yourself, but be respectful to her.