Because that article doesn't refer to the teachers directly. This sort of 'das' describes a group of things or persons you could draw a circle around and speak of the circled as a whole. Das sind meine Kinder, das ist meine Frau, das sind meine Häuser und das meine Kreditkarten = These are my children, this is my wife, those are my houses and those are my credit cards. I don't know if there is any specified rule for this case. It's similar to 'es', but more definite, like 'it is about time' - 'es ist zeit', the es/it doesn't have much of a meaning and is more like a little helper.
I've found this for you: http://en.wikipedia.org/wiki/Predicate_(grammar)#Predicates_in_traditional_grammar
Our sentence here uses a predicative nominal and the german wikipedia says, they only follow these verbs or copulas respetively: sein, werden, heißen, scheinen (zu sein), bleiben, gelten (als), (sich) fühlen (als), (sich) dünken (als), (sich) erweisen (als), (sich) entpuppen (als), sich glauben (als). And for the verb's conjunction, canoonet says: 'If subject and predicative nominative are not of equal number, the verb [copula] is in general used in the plural form'. If you compare that to my examples, everything is explained and makes a lot of sense to me.