Is it legal to bring a Bible to school and talk to other kids about Christianity? If not, how does a school have the right to take away a right that would bring others to heaven?

Yes.  Bringing a Bible to school and talking to other students about one's religious faith in a public school is protected by the First Amendment of the U.S. Constitution.  The school may impose reasonable time, place and manner restrictions on student speech activities, but the restrictions must seve a valid purpose and not unreasonably restrict the ability of students to express themselves.