I agree that both are correct. In my experience, "when I leave" is more common and a little more natural in this case. Our perspective stays in the present. You could say "after/once I've left" which also sounds natural. Here, our perspective shifts to the future - the time of leaving school when we can say that we have left school.

Both are correct. Both are natural. There's almost no difference in meaning. There's a tiny difference in the feeling or the point of view. "I'll continue studying in a university when I leave school" emphasizes continuity. You are thinking of it as a continuous, connected education. In your mind, nothing important happens between leaving school and entering the university--it's one event. Of course we know there must be a gap, maybe even a whole summer in between, but it's not important to the connected story of your education. "I'll continue studying in a university when I've left school" emphasizes a break. You must first leave school, that's one event. You complete the act of leaving school. You go through the graduation ceremony, say goodbye to your classmates, sign their yearbooks, all the rest. Then you enter the university, that's a second event. There's no real difference in what's happening in the real world, but a slight difference in the way your are thinking about it.

I would like to understand one thing: is it correct to use the present perfect tense after the word "when" in English? Or is it better to use present simple? I think using the former is more appropriate , even though I am still confused about its correctness . For example: I'll continue TO STUDY at university AFTER I LEAVE THIS school. Or I'll continue WITH my STUDIES AT a university when I leave school? Which one is correct?

i don't know what is technically correct but I would use either of them I would swap the "in" for and "at" in both cases I'll continue studying at a university when I've left school. I'll continue studying at a university when I leave school.