I doubt the school bus could clearly pick up what you said. So you have two options:
1. Deny it completely. Say he's been harrasing you and you threatened to tell a teacher but he ran and told on you.
2. Suck it up and admit it.
Trust me it's not a big deal detentions don't effect you getting into a good college/university, well it doesn't in England. Plus it's a nice time to relax and plan out your holidays.
I think I had about 10 detentions in my secondary school 8 for not doing homework and 2 for fighting (both the same kid we just hated each other no reason for it just did). Only ever went to 1 but all schools are different.