How unhealthy would eating a 500g container of creamy coleslaw be?!

Unhealthy! probably contains a lot of fat, but it isn't going to kill you just as a one-off.

You could always microwave a jacket potato and have that with some coleslaw: not much work involved for that....!

500g!!! eeeuuuuw you will be passing gas for 2 days your rear will not thank you for putting that amount of cabbage through your intestines all at once. There is also a lot of mayo in 500g not to mention if there is cheese and or onion added. Just buy a fresh roll on the way to school and dump a couple of teasoons in but I dont advise eating 500g in one go.

unhealthy and contains quite abit of fat, but i assume it has less fat in then say what mayo has.