There is little difference between with/ in hindsight. 'with', implies that I am now in possession of information that allows me to reflect more meaningfully on the past. 'in', implies that I am reconsidering my position on this matter at the moment of speaking: i.e. now. - With hindsight, I realise that I married the wrong person. - In hindsight, I suppose it would have been better if I had got divorced earlier.

I believe 'at school' is more commonly used in the U.K., whereas 'in school: is more generally used in the U.S. I'm not sure about other English speaking countries, though.

Hi Jessica, There are as many examples of errors on the internet as there examples of correct language use. Since language in alive and always evolving and even some native speakers make erros themselves or play with language for fun, this makes rule-making more of a shifting landscape. Last year, I lifted a whole section of Phrasal verb exercises of a English learning Spanish based website which I intended to use, but had to alter ALL of it to the correct English, which I corrected for British AND American formal and casual forms. That said, I would write and generally say, "With hindsight, Laura realised she should have studied harder in school." (...and the comma matters, too.) Poor Laura.

Hindsight is (time)-in. In time. School is a (place)-at. At School. You can say at or in school. Either would be fine. EG: I should have studied harder in school. I partied too much when I was at school. I met my wife at school. I made good grades in school. What is your primary language?