That really sucks...I would be really pissed...and if I knew who they were I wouldn't just let it go...although I'm not 18 yet so I wouldn't be in nearly as much trouble as you if I did something about it.
My Grandfather had a '47 or '48 Chrysler Limosine. It was black and had the original paint on it. The paint wasn't in the best shape (being 55 years old at the time he owned the car) so he would wax it like every week with black wax to try to keep it looking good. Well, someone egged it one night and when he got to his shop in the morning (where he kept the car) he found it. Luckily the 50 coats of wax protected the paint so much that the eggs didn't do anything! He was still pissed but it didn't mess up the paint. I would suggest maybe waxing your car a lot to at least attempt to protect it from them doing it again.
Just a suggestion.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!