I think OP says not "leaving" as she's looking for nice memories, and leaving is a bit predictable.
I can think of a few nice ones.
Coming back after the morning off for a music exam in year 6 to find the head telling me that I was invited to the reception form Teddy Bear's picnic. I helped in reception form at lunch three days a week, and the rest of the form was madly envious.
I didn't really have a best friend at primary and sort of drifted not being particularly friendly with anyone, nor anyone generally being a problem. When we were going on the residential in year 6, they decided to sort the dorms by asking random people to name who they wanted in their dorm. First girl asked chose me in one of her four. I was so proud to be chosen by her as well as amazed.
Sing-songs at the end of term with the whole school, followed by three cheers for the teachers. one teacher always came out with cotton wool in his ears because he said it was too loud.
A school trip in year 2 to a windmill.
Some of the school plays we did. Especially the Christmas show.
At Secondary: Lots of memories with friends like sports day (none of us was ever in it) sitting on the field chatting in the sun. I had a lovely group of friends who generally were very kind and inclusive people.
House debating used to be very funny, particularly the final, which had the added advantage it was over lessons.
Some of the charity concerts the 6th form ran ("Head girl and the seven prefects" pantomime I think was one of them)
Our Year 7 English teacher who had a funny way of teaching, but I can still remember some of his lessons almost word for word. "Fish's ruler like black pudding" was one such lesson. "Get that Walrus out of my bathroom!" another... Amazingly I can even remember what he taught too.
6th form. Not as much fun really. I changed, which was probably a mistake, but parents thought it was a better school (wasn't, they were just better at window dressing and pulling wool over parents' eyes)
I remember one of the boys putting another one through a window messing about (neither hurt, one who went through thought it hilarious) and then the story got up to tell to the teachers to explain it-they clearly didn't believe it, but accepted it with two statements: "Why did you choose today to do it?" (it was open afternoon for prospective parents) and "Right, I've accepted that, can you tell me what actually happened now?" (we stuck to it)
The fire alarm that was accidently set off by two of the 6th formers trying to learn to juggle and no one thought to tell the fire brigade before they'd turned up-4 engines worth. And then they found the fire alarm system had been wired wrongly so the alarm was showing the wrong alert button had been pressed so they had to check the whole school before letting us back in.
We had some fun in what was grandly called our "study" designed for 4 people, ours crammed 10 in. We had the Christmas party where one of the girls bought some home made wine in, and we sprayed silly string everywhere and bust the hoover trying to clean up.
Then it snowed and we had a girl from Hong Kong who'd not seen snow before and we were banned on going out, so we collected snow from outside so she could build a snowman. Once it was melted we used the one remaining hoover to try and remove the evidence and, unsurprisingly, bust that one too...
I was lucky in school in that my years were generally nice people with a few exceptions, but I tended not to come across the exceptions too much.