it is like one trousers two trousers, we would say one trouser two trousers like Christian mentioned, there is no Plural the 'erer' in 'Lehrerer' sounds really strange Wasser, Erfinder, Entdecker but Bauer (farmer), here you say ein Bauer, zwei Bauern
Does die fünf Lehrer mean 5 male teachers or could it mean a group of male and female teachers ?
I believe this would refer to a group of teachers without specifying gender. I think there may be a way to refer to a group of teachers that is exclusively female (Die Lehrerinnen perhaps?) but so far it seems to me that using the plural masculine form would be used to refer to a group that has both male and female. I'm only a beginner so I could be wrong :P but I hope this helps.