...there was once a time when children could be expelled from school if they didn't salute the flag?
Speech / Religion
Several schools had this rule in the 1930s. Among the children expelled were many who belonged to the religious group Jehovah's Witness. In 1940 the Supreme Court upheld the schools rights to do this (Minersville School District v. Gobitis, 1940).
The Supreme Court reversed its rule in 1943 (West Virginia State Board of Education v. Barnette, 1943).
The First Amendment wasn't always first. There were 2 other amendments before it in the original Bill of Rights, but those two were not ratified.