One student pats him on the head before entering the room. The new arrivals begin slouching about and making more jokes. The principal and year level co-ordinator arrive. While the teachers may be tempted to send the students home, the students are drunk and their judgment will be impaired. The students are at a greater risk than normal of injuring themselves, or getting into trouble.
People can become violent when drunk.
So, as with any case of drug misuse, confrontation must be avoided. The normal relationship between teacher and student has been broken. Teachers need to be tactful, and to avoid direct displays of their power. If there are facilities and staff, the students should be separated and put in a dark and quiet place. Stimulants such as coffee won't counteract the effects of alcohol.