More than likely they just don't want to see you get caught up in working there forever, without ever shooting for your dream job, whatever that may be. If you've been showing an interest in the medical field, they may be trying to help push you toward going for a medical degree, so that you can climb the ranks in their hospital. There are a huge multitude of reasons why they could be suggesting you return to school, depending on a variety of factors from how close you all are as co-workers to their own needs for staffing.
If it bothers you enough to make you worry about your job security, you can always ask them if they feel like you are not fit for your position with them (but in a way that isn't "what, you don't want me here?" you know?). Or you could ask them for suggestions; what did they wish they had gone to school for? Is their job rewarding? Etc. A lot of times senior staff see's a young person with their life ahead of them and it stirs up old dreams in them. It really could be as simple as some of the staff really wanted to be an astronaut or an artist. If you're trying to figure out what you want to do, it is never a bad decision to ask around and see what sounds good to you. The fact that you've been with them for two years tells me that they value you, and that you're secure in your position. They really probably just want to see you do the best with your life possible, which shows that they care.