Is it self-defense when he (and the rest of Hogwarts) could have retreated to a safe place instead?

The rules of self defense are slightly different in war
That was the most defendable position, and given that Harry needed to find something there to stop you know who, it was necessary that they should defend there
The bad bois would've come after him to kill him anywhere he went, and if he just ran and hid, snake nose boi would've kept killing people

But was it legally a war by UK law in 1990s, or just a brawl?

the wizarding world is different

No.

Can you elaborate on your answer? :)

Maybe. I mean I’m tired. Maybe in the morning.

Well, if he and Ron went off to jail, then so would my ginger and my nerdy classmates.

(Get it? They look exactly like Ron and Harry)