Guys I'm confused. In Part III Marshal Strickland says to his son "Remember the word disipline" and Mr Strickland is principal of Hill Valley High School in the Future. Is it his son? I thought so at first then I realised if it WAS him he'd still be alive and over 100 years old?!

No, actually, Mr. Strickland would be Marshall Strickland's grandson.

That would have made sense