You DO sell Amway on company time if you work for Amway. The schools are paid for by the taxpayers. Therefore, it should be up to the taxpayers to decide what is taught there. Are you prepared to put thie question to a vote of the taxpayers?
If you are against using the schools to proselytize, then I take it you are also against using the schools to push evolution, gay rights, socialism, etc. Right? If it's wrong for Christians to use the school to teach things that support Christianity, then it must be equally wrong for non-Christians to use the schools to teach things critical of Christianity.