You can do that in the ministry as well as at school. Some find it difficult to preach to their peers at school. You likely can understand why.
لٰكِنَّكَ قَدْ تَسْتَصْعِبُ أَنْ تُبَشِّرَ رِفَاقَكَ فِي ٱلْمَدْرَسَةِ لِأَنَّكَ لَا تَعْرِفُ مَاذَا سَتَكُونُ رَدَّةُ فِعْلِهِمْ .‏