How can one invite to Islam in a Christian school given their rejection of the call to God?

The means of دعوت (Dawah - calling to Islam) are not limited to specific forms. Dawah can be done individually and secretly through personal dialogue and good argumentation, as Allah Almighty said: (Say, "I advise you of one thing: that you stand up for Allah, in pairs and individually, and then reflect.") [Saba': 46]. Focus on mentioning the virtues of Islam, while criticizing false religions without explicitly naming them. This form of Dawah requires extensive knowledge. If you are restricted, then be proud of Islam and hold your head high because of it. Be a good example through your righteousness and adherence to the religion, outwardly and inwardly.