Going against the grain here & I'm normally all for resilience & just getting on with it.
Same thing happened to my son. Was in a tight knit group of around 8 friends all split evenly between 2 classes - all good.
Then they were mixed- 7 in one class & just my DS in the other. I spoke to the school & was reassured like you were. I was positive about it, was sure it would be fine, lessons are just for learning, youll see them at break, it will be fine etc. But it wasn't.
DS increasingly was left out, not deliberately but because he wasn't around them as much so they forgot about him. Lessons ran late, or were at opposite ends of the school so frequently didn't meet at break, school trips were done by class so he couldn't sit with his friends on the coach, activities on residential again sorted by class so he wasn't able to do them with his friends either. Social plans sorted in lessons and he didn't know about them.
None of it was deliberate, then were just young & thoughtless & he was definitely wanted as their friend but they grew apart.
Being in separate classes didn't result in him widening his friendship group as he was particularly sporty (& so we're his friends) & the other boys in his classes were into computers etc which wasn't an interest of his at all.
I spent a lot of my time messaging the other mums, arranging get togethers, sleepovers, lifts to practice etc so he was still included in the wider group.
When they got Snapchat around 13 it improved massively as they used this to arrange their social life & didn't mention it at school so much.
They are now 18 & all away on their first lads holidays so it did work out, but only because I had to involve myself so much with the other mums at the start.
The biggest problem was that all of them apart from him were in another class - it wouldn't have been an issue if they were more separated and spread through different classes.
I've now got a much younger DC and am adamant that I will not let him be separated from all his friends in the way DC1 was at school. Fine if they are separated evenly but not all of them in 1 class and my DC in another. It really wasn't positive for my DC and I would definitely be 'that' parent if it was to happen again.
Good luck.