They should be able to have relationships.. What consenting adults do with each other is their own business. If the relationship interfers with the teaching of the teacher well then he learns a valuable lesson.
I know plenty of teachers who married their students and are quite happy. Love doesn't distinguish between position really.
Also I would hope that most 28ish year olds would have the decency or actually the taste not to date willing 16-19 year olds even if their ''legal''