The two have different senses.
If you slope off, then you leave somewhere/something that you have already turned up at. If you went to a party but you decided to leave early - you slope off at 10pm.
If you bunk off school, you don't go in at all. You might leave the house in your school uniform but you never actually arrive at school. You spend the whole day somewhere else, probably with some friends, and then you go home later in the day. Your parents think you've been to school but you haven't been anywhere near the building!
To slope off, you have to have been in attendance in the first place.