It seems he knew you were pregnant when originally mentioning a promotion. (even if he didn't know about twins)

There is nothing illegal about deciding to have you interview for the position. It seems the other teacher
also has to now interview along with a 3rd candidate. Unless you have a binding contract guaranteeing
you the promotion, the employer did nothing illegal here.

You can't be discriminated against due to your pregnancy or any other reason prohibited by law but that
doesn't seem to be the case here.