Hi GMAC,
I presume that turning 18 in May means that you are a junior going into your senior year. This is the right time to be thinking about a harmless summer fling with slyness. Your subject line says a lot. Take my word for it, once the thought is in your head, it won't go away until you just do it.
I wish I had done so at that point in life, but in my day ('60's), hair was "the thing", and although some guys had "summer crewcuts", nobody shaved their heads.
Shave it right after school gets out and make it a goal to keep it shaved all summer. If you don't like it, you can always change your mind, and it will be back to a #1 in a few days.
If you keep a #1 for a couple weeks before shaving, your scalp will get enough sun that it won't be too white when you shave, and it will even out in a few days.
As for the girls, you know they go in all directions. That is as true for 17 years old as it is for 47 years old. Some will like the look, some will like the smoothness, some will like the "bad boy" image, and some will NOT like it at all.
Go ahead, have a god time...