Am I more likely to make life long friends in secondary school or univeristy?

Basically what the title says. I'm at the tail end of Y13 and I love my friends but admittedly we've started to drift as of late. 2 of my best friends, one of which has drifted a bit due to finding other friends that he has become very close with (but he still considers me a good friend) and the other has gotten closer to me and I know that I am his best friend and he is the same for me. The thing is I know that we're likely to drift after we finish as we are all busy people and are very school focused so we don't hang out much outside of school (even though I'm gonna try and push for us to do a bunch of hangouts over the coming summer and we've agreed to meet up once a year after graduation).
And because of this, I feel kind of like I've blown my shot at making life long friends at secondary and since a lot of people say that uni friends are for life, I am hoping to make long lasting friendships there. But I am worried about other factors such as uni only being around 3 years for most (I'm planning on a 5 year course but most people I meet will probably leave by then) and that everyone will go back to there hometowns after graduation.
Those factors above have led a bunch of people that I've seen to say that secondary school friendships are actually the life long ones so I'm wondering at which school are you more likely to make long lasting friendships.