Katherine, my DS has also had various peculiar habits and tics over the last 2 or 3 years (he's now 5), ranging from excessive blinking, throat clearing, lip sucking, strange mouthing and so on.
Initially I tend to ignore it, sometimes it goes away all by itself. When it has reached the point where it's getting on my nerves, I have been a very cruel mummy and asked him why he does it and that it looks a bit daft and perhaps he should try and stop himself doing it. Just by pointing it out to him, it makes him aware of it and gradually it goes away. In fact until I read this thread tonight I'd forgotten about his last habit but on reflection he hasn't done it for a while, so that's another episode passed.
DS is a fairly sensitive, stressy individual, but I haven't noticed a clear pattern of when he does these things, often it's when he's tired, sometimes it's before he's about to speak so I wonder if it's a lack of confidence.
I remember reading somewhere that it's very common at this age - if school aren't helpful, how about a quick phonecall to your health visitor?
Anyway, I've never worried unduly, each phase has passed just as I'm beginning to wonder if it's a habit for life.
Hope this sets your mind at rest.