I've lived in Portsmouth all my life. I'm 17. I don't live on Portsea island, but I still have a PO Postcode.
It's safe i'd say. There are fights at clubs. There are the roaming chavs going "GOT A FAG MATE?" "no I don't smoke" "YE YOU DO YOU <insert random collection of swear words>". True of any big city.
My friend from another forum went to Portsmouth Uni and was fine. The places a person goes are typically fine, southsea common, beach, guildhall walk, liquid, shops, high street all that jazz. It's when you go into all the little crap parks, council estates etc that it's get rough, but is that not unlike any city?
The places I become more alert/avoid are Leigh park & Paulsgrove if you venture further than the island. They're rough areas populated by chavs living on benefits.
Fratton can be a bit dodgy at night, found that out one night going to the train station after a party. Looked a guy in the eyes and he started getting lary and following us to the trainstation, where his little croney bottled one of my mates.
It's not that bad for a student though. The friend I described earlier, he said he loved it and can't wait to come back