"Voluntary" Islamic prayer in school
You've lived in a quiet suburb in Michigan all your life. You like it there. You have many friends. Your son is on the high school football team. Over the years, there's been more and more Muslims move in, but that's all right: they're nice people and you get along just fine. Recently, the Muslim students began to outnumber the Christian population at your school, but again, that's all right: you're ok with that. Then school hired a new coach for the football team, and he's beginning to seem … not all right.
He's Islamic, but that's not really the trouble. The problem is that he's evangelistically Islamic. He's always mentioning Allah to the kids, and bringing up verses from the Koran to illustrate points. Now he's started doing prayers after games. The team is 95% Muslim students, and none of them complain of course, but your son is Christian, and he feels pressure to go along with the team.
You and your son are not "offended" by Islamic prayer, you just think it shouldn't be forced on kids in a public school. This is on public school grounds, by a public school official, during a public school event, and that makes it a unique situation. This is not like having a private room to pray, or dedicating a silent time, or a religious group using school grounds after hours, this is your son's coach during a public school event forcing your son to decide to join the rest of the team or make himself a known religious minority.
The coach (a government employee) decides to have a "totally voluntary" prayer session on the field and he and 95% of the team start praying in unison. Your kid, who everyone may or may not know is Christian, is now in the position of setting himself apart from the team and spectators and defying his coach, who is in charge of determining who plays the cool positions, and even who plays at all. It's so close to coercion it's probably best to legally consider it de-facto government promotion of Islam.
Luckily, we live in the United States of America, which has the First Amendment, which has been broadly interpreted to prevent government employees, such as teachers and coaches, from promoting their religion to students, so this scenario could never actually happen! You don't need to worry about this entirely hypothetical concern.