Bully

I believe the counter for this trophy will only go up if you get "Theft" added to your trouble meter when you take the bike.
Taking a bike you unlocked from various racks around the world map won't count towards this.

The best way to be sure your theft will count towards this is if you see someone riding a bicycle on the road, just run up to them and press Triangle and this will throw them off the bike and press Triangle again to get on and steal it.

There is usually always someone riding a bike outside the gates of Bullworth academy, just make this something you do on your travels while playing the main game. Also you'll almost certainly need a bike for the missions you're attempting at the time anyway so this will help you do this trophy at the same time.

Just be careful that you don't throw little children off bikes because otherwise your trouble meter will be maxed out and you will have a lot of cops/prefects to evade capture from which will add trouble to the mission you're doing!