How to keep track of students entering and exiting a clinic.

Hello everyone - I wonder if you guys could offer advice on offering a solution
to this clients need:
My customer is a school of about 250 adult students, who go to school for three
to four years to get the diploma they are working towards.
As part of the studies, they need to log a certain number of hours per week at
the clinic.
Management is requesting a mechanism that simply records time in and time out for
each student, preferably with a biometric scan.They also want to do this on the cheap, so they want something very basic.
What do you think would be the most cost-effective way to give them a monthly
log of students coming in and out?
BY THE WAY: They have now building security doors to speak of so I told them the system would have no way of knowing whether they were doing the work, or even knowing if they were in the building in between check in or checkout. Still, they want at least to know that they were there to clock in and out.