Do you use a refillable plastic water bottle during class? You may want to consider throwing it away.
Polycarbonate plastic is durable, hard, and clear, so it is often used in sports water bottles as well as baby bottles. The problem is that this type of plastic contains a chemical called bisphenol-a (or BPA for short) which has been linked to unhealthy hormone changes in animals. The effect on humans is still unknown, but there's no reason to believe we're safe.
The Canadian government has recently declared BPA a toxic chemical. In response to this, Nalgene has stopped making these potentially unsafe bottles and has replaced them with bottles made from a new BPA-free plastic. (They maintain that BPA is safe and state the change is only due to customer demand.)
Your best bet? Play it safe and swap your old plastic bottle out for a newer one. Better yet, get one made of stainless steel.