If you are approaching random women that you've never seen before, cold approaching is generally very difficult and will yield a low success rate, disregarding ones clearly wanting you to approach. Most of these women who you are approaching are probably so focused on whatever it is they are doing (going to class, having lunch, etc.) and are not exactly expecting a guy to come up and proposition them. Don't take any of this as if you are doing something wrong, a high rejection rate is normal.
Now, this is all assuming that you are applying a bit of common sense and not approaching women who are clearly trying to shut off the outside world, which I'm sure you are already aware of this.
The reality is though that you will have far better success approaching women that are in environments where you may become a familiar face and where socializing is normal. Class, gym, social circle, parties, clubs, bars, etc.
Most guys can't do this, I say you should definitely continue to approach women you see that you are interested in as long as you are applying some common sense and able to be genuine and non threatening/awkward. You are in an environment that allows for so many opportunities to meet women, make the most of your time there because once it's over you'll find how much more difficult it is in the "real world".