How hard is first yr uni writing compared to gr 12?

Hi everyone! I'm in gr 12 going into accounting next year and I was just wondering, how well does your writing really have to be? Like in Gr 11, I thought my writing was pretty weak compared to my classmates, but my teacher was pretty lenient and I ended with a 92 (class median was an 85). How much harder is english writing in uni compared to hs? Any advice on how to improve writing, in order to be prepared for 1st yr?