Best way to tackle this one is if you have given a girl flowers, then you will unlock the kiss interaction eventually on the L1 + X option (marked by a lip symbol).
You can then proceed to kiss this girl as much as you like by repeatedly interacting with her with L1 + X provided you don't get interrupted by other pupils wanting to fight with you.
You can also kiss boys to get this trophy (the Xbox 360 Scholarship version actually had an achievement specifically for kissing 25 boys).