To Enrol or To Enroll -- That is the Question!

I went to buy milk for the kids at Pioneer Center (since we had lunch at my folks' place). On my way out of the exit gate, a tarpaulin announcement caught my attention. It was from a pre-school (I think) inviting parents to bring their kids there. Normally, I would dismiss those kinds of advertisements, but the footer of this one somehow caught my eye. It was a basic call-to-action for parents to "enrol" their kids.

My knee-jerk reaction was -- that's a wrong spelling! It should be enroll. How can a school, no less, make a mistake in its own advertisement? Its just not acceptable for a place where children learn to spell to not know how to spell itself. But was it wrong? Is it really enrol or enroll?

Well, it sort of bothered me quite a bit, so much so that when I got home, I had to check the dictionary. And what do you know -- both enrol and enroll are correct! Turns out that the word can be spelled with either one or two el's. And it follows that enrolment and enrollment are also acceptable.