Each tower had a number of years, so I could see how you got that 70 to each tower.
Yes, but I think the pp's point was that we're told there are ten first formers in the North Tower dormitory that Darrell sleeps in, therefore there are a minimum of ten North Tower first formers. But Darrell says there are 25 to 30 girls in her form, which means that there can only be fifteen or twenty girls from the other three towers combined.
I mean, obviously EB had zero interest in anyone who wasn't in North Tower, anyway and the weight of NT numbers maybe explains why no one who wasn't in NT got to be head girl of a form etc 
I don't think who moves up a form and who doesn't makes any sense as a form of streaming, mostly. Darrell does reasonably well in her first term and moves up to the second form along with the girls who've been there two terms longer than her, but Gwen is very weak academically, and she moves up too -- while Felicity, whom we're told finds first form work very easy because she was at a 'good prep school', and brilliantly clever June join MT in the summer term like Darrell, but are kept in first form for another academic year, apparently, along with their cohort. (Or at least, they're all still first formers after the summer holidays.)
And yes, we're told clever Alicia is allowed into the fifth form because she can work for the School Cert independently after she fails due to illness, but Gwen, who also fails, is similarly allowed to progress with no such explanation, though Connie is kept in the Fourth to have another shot.
I mean, clearly EB just wanted to keep Gwen around as a Bad Example...