We were surprised that a teacher from my son's school asked him not to say "oh my God," but instead to say "oh my gosh". I think that is based in the Catholic faith because they do not like the God's name to be said in vain or so frequently especially with anger, play, or inappropriate situations. I think this would be good practice to some extent. The other reason might be because of some atheists who ask for their rights by not mentioning the name of God. I also know that here, one cannot say any religious words at work, school, etc. I think I'm going to teach my son that due to the need to not say the word God in any time especially inappropriately, he can switch it with the word 'gosh' that does not mean anything. This way, he can maintain respect for God's and praise we have.
It is wise to try to objectively see the situation from the perspective of the other, just as you are doing. In the public arena, many schools will try and restrict students from using these phrases which include references to God, including "God bless you," when someone sneezes, etc. In parochial school settings, they caution their students from saying phrases like "O my God" because they do not want the children to learn to use God's name in vain. This is understandable. We also do likewise. Speak to your son regarding his intentions when he uses these expressions. Show him the many psalms that use this phrase in prayer, especially Psalm 22, which the Lord Jesus Christ prayed on the cross and explain to your son the power of this short prayer. He should not feel intimidated by school officials if he is being reverent. They cannot impose their lack of religious neutrality on him. Even in public schools, if a student wants to bow his head and pray before lunch, no one can really stop him, but he does not have to be so obvious as to attract attention for the sake of attracting attention. If what your son is saying is only a matter of speech and used as an exclamation to emphasize a dramatic feeling, then he should not use this prayer in these types of contexts, and can say something else appropriate to express his emotions.