I enjoyed them at the time, and I still think they're good as an adult. I have them all, I think, including the 5th form friendship one which apparently is hard to find.
Dd1 enjoyed them much more than the Chalet School, what irritated her about them was the boy chatter. I also thought as a child that the school was remarkedly relaxed about their boarders going off. "Oh I've met this lad I knew a long time ago who's at a different school, I can go to a party with him, he'll bring me home" "Oh that's a lovely idea, don't be too late..." Hmm. I can't see any school, even in the 80s being that relaxed. I think for the boarders at our school it would have been a
look and a "ha ha very funny, good try" response, and they weren't that rigid. Maybe I'm wrong on that. I don't think the tennis was that realistic either, she'd have had to be training a heck of a lot harder to get anywhere near what she was achieving.
I think what makes the Trebizon to me, is that they aren't just Rebecca wins lots of tennis and nasty girl is jealous stories. Each story is different. You have mysteries, boy trouble, hauntings, difficult decisions, friendship stealing etc. But the main group stick together, with (I think it's) Jocalyn on the outskirts of it, and see things through generally.
Rebecca isn't that interesting, but the whole group together are: I can't remember them off hand, but there's Tish (very bright and sporty, generally the leader), Sue (Musical), Mara (I think, very rich and supersticious-is she Greek or somethng?), Elfie (who thinks she's fat, but you always get the impression isn't) and another rather nondescript Mary or something?
I wouldn't pick them up as a comfort read like I would the other school stories, but I do think there is a place for them. Not sure they'll survive as long as the CS etc. but I don't think they're completely gone yet.
And I think it's notable that Ann Digby is one of the writers that was asked to continue either Mallory Towers or the St Claires. Although I think she repeats one of the stories from the Friendships book about twins in one of the ones she rewrites, which is a bit poor, as it wasn't that brilliant a plot in the first place and the rewrite is worse.