I'd advise you to
1) Text your old school friends - it would be nice to keep in contact, even it's via text
good skill to have, as when you leave uni you'll have to keep in contact with uni friends somehow
2) Join societies - and before you leave them, PLAN something with a group, or one or two people. That's how most people make friends, after lectures/society socials and are able to bond better. Even if it's just one person
you'll be introduced to many friends of friends, and that's how you will make more friends
3) You say you haven't been invited to nights out. Yet again, maybe you should do that. When there's a student night out, text bunch of friends from the same course or society and see who's up for it. Make it obvious that it's a group text i.e. start with "Hello everyone! So I was wondering, who's up for...". You'll get few no/yes and some maybes (this group generally wants to know who else is turning up - if there are few they know, they might come along).
Even if 3) doesn't work out, the ones you texted know you're up for going out, so when they get invited by somebody else, they might invite you also
hope this helps.