Henna and indigo might be good possibilities for you, which can darken hair gradually. There's a website up at hennaforhair.com that's got lots of information.
I used to go to catholic school as well -- 12 years of it -- and while we couldn't dye it what they called "unnatural colors," the standard human color palette wasn't off-limits. That's a shame. :-(
if your hair's naturally quite light then you can go for the lighter version of Henna which is Cassia - that's definitely worth a try. A friend of mine used it on her very white blonde locks and it darkened them only very slightly - her hair became a more golden light blonde.
You can also do Henna glosses instead of full out Henna dyes - which is mentioned on the hennaforhair.com website that Janis gave you... it'll do like a colour build up rather than dye your hair huge amounts in each go - it'll take about/roughly 3 henna glosses for the hair colour to stick and get darker.
hope that helped a little :-)