The role of school education in a child's life is much broader than that. Schools have to provide an academic education to students in things like maths, English, science and the arts, educate them in general life skills via things like home economics, PE and technical and also further their social and moral development via things like RE, social education, sex education and other similar classes.
You need to have all of those things to operate in society as a normal functioning human being. For a subject like RE (at my school at least), it wasn't about telling children that they should follow a particular religion, but rather about providing information about a variety of religions and their practices. Sex education (incorporating LGBT) is the same - you are teaching children about sex, relationships and everything associated with that, and it would be very strange given the society we live in if that didn't include a discussion about LGBT topics.
Having LGBT education isn't going to suddenly make kids gay, but it might make life easier for the ones that are.