Now, this took some getting used to when I first started teaching in South Africa, where they use the same system. Why? Because when/where I was a child, a checkmark (AmE for ✓) on your work meant that you got it wrong. This is actually fairly counterintuitive, because a ✓ can mean 'good' in various other contexts. For instance, if I wrote an essay in school and it was just OK, it would get a ✓ at the top of the page. If it were (usual BrE = was--but we'll get to the subjunctive some other time) very good, it would receive a ✓+ or ✓++. And a ✓ in an advertisement or on a grocery list means 'we've got it' or 'mission accomplished' or similarly positive things.
It seems that many American schools use the British system of ✓ for 'correct' and X for 'incorrect', while calling them check(mark)s and exes still. It's not clear to me whether this is a recent innovation or a long-standing variation. (American readers--did you get checked or exed wrong, and when?) But there is some evidence that the system that I knew as a youngster is still around in some places, as these teachers (on the page linked above) note:
Canuck: I am a Canadian teacher, working in Korea at an American school. (Yikes) As a result, I'm confused! I've always used a check mark for correct answers and an x for wrong. However, my students are confused and think this is backwards. [...]Over on the Guardian's Notes and Queries page, it's noted that the Swedes also use ✓ to mean 'incorrect' (adding to the multitude of reasons that I feel a kinship with Swedish culture), and it's supposed that ✓ originally came from V for Latin veritas 'truth'.
Wig [from western Michigan]: I wonder how much it has to do with how papers were graded when you were in school? It's a good question, but everyone in my school uses a checkmark if it is wrong.
During the first multiracial elections in South Africa (which I was lucky enough to witness), trainers crossed the country teaching people how to mark a ballot paper. One of the things that they had to contend with was the fact that people with some schooling saw the X as a sign of wrongness, so rather than putting a cross/X next to the person they wanted to vote for, it was some people's urge to put crosses next to all the people they didn't want to vote for. So, it's not just the tick/checkmark that can sometimes mean 'wrong' and sometimes mean 'right'--the cross/X can too.