Gingejbee is right, it's all about context.
The English tend to say: I was at The Grammar School, rather than in it.
But, we could say, I was at school in the 1950s, and if looking for an alibi, might say 'I was in school when the crime was committed').
An older-fashioned form might be 'I was up at Oxford', meaning attending a college there, although it could mean I had gone to Oxford for something else.
Blimey, Mohammadihamed, it really does get complicated, and is highly dependent on context (and, in the Oxford example, on who is saying it!)
Your cited example "Jim and Sal fell in love in high school, and got married after graduation" is fine as it is - you wouldn't use 'at high school' in this context.
Sorry you asked?
Allen