I am in the same boat but a little simpler. You will need at least UHF tags to allow the range you require then you need three readers/interrogators... then you will need to develop the software to perform triangulation to locate the exact co-ordinates of the asset. There are a number of different ways this is done but the most used today is DTOA differential time of arrival... this should get you started... you will need to extend on the software then to calculate the attendence with reference to a database of the students.