Bully

In order to kiss a girl, or a boy, you must have completed class Art 1, as this will reward you with the ability to present gifts to girls in exchange for a kiss. After gifting certain girls with gifts, e.g. flowers and chocolates, you will be presented with the ability to kiss them. You must first find a senior girl and get them to like you. This can happen either through natural progress or talking to girls and giving them compliments. If you have completed class Art 5, you skip the need of having to first give a girl a gift before you can kiss them. There are also generic senior girls that wear green skirts and white tops which can also be kissed.

In order to talk to a girl, lock onto them by holding . You will then need to keep pressing until the option to gift them appears. Depending on the girl this will either be a box of chocolates or a bunch of flowers. Once you have gifted them, you will then be able to kiss them. You are free to kiss the same girl many times over, allowing you to essentially grind this trophy out. However, I suggest that you work towards this trophy as you work towards the Green Thumbs Up trophy as you need to gift a significant number of flowers anyway.

The below image shows what the dialogue option of kissing a girl looks like once the option becomes available:
 

You can track your progress for this trophy in the Stats menu by pressing and then navigating to Stats > Social > Romance > Kisses Received from Ladies / Kisses Received from Gents.