What should a teacher watch out for if a student in his class has diabetes.?

Depends on age. First, the child with diabetes should be treated like all other kids and not be socially isolated or pointed out in any way. The principal issue would be too much Insulin which would cause a low sugar faint. If this happens, the child needs a sweet liquid (not diet) like cola to get the sugar back. Diabetes out of control will cause inattention, lethargy but is not immediately dangerous.