It's a sin for Christians to use the lord's name in vain and some will get offended when other people do it. Generally it's socially acceptable in America but it's kinda rude to be going around spouting religious epitaths. I'm atheist now but I was raised Catholic. When people at work yell out stuff like "Jesus Christ!" It still sounds like they are dropping the F-bomb to me. It's not really appropriate for school or work. This might be a good time to break this habit. Imagine the shit show if he replaced the word "God" with "Allah" or was yelling "oy vey." If he's getting in trouble at school for it he should probably stop.