The university now has paid sick leave for GTA's and GRA's.  We don't yet have a College of Science policy but here is what we are doing in the Dept. 

In the past students had to find a substitute and then make up their hours. Now, if you have sick leave accrued, you can use that and your replacement will be paid - so no need to do a makeup. You should have a section in your time sheet to both declare that you took sick leave or that you worked extra hours in support of a sick colleague.
As before things work much better if you can help your instructor find someone to replace you. So if you are sick, don't tough it out!!! take your sick leave!!! Almost all of you already have several months worth of accrual available to you. But we'd appreciate your help if you can try to identify a good replacement and inform Kelly Carter and your lead instructor. If you cannot identify a replacement, please tell Kelly and your instructor so that they can find someone. If you are so sick you can't even inform anyone, we'll figure something out but will need you to tell us that you took the leave and work with Kelly to get it on your time sheet. You should talk to your lead instructor for specifics on how one deals with this for each class. So, in practice, things will work pretty much as they have before, with things going most smoothly if you have an identified replacement. The change is that your replacement will get paid at an hourly rate (set by University wide policy) and you are not on the hook for makeup time unless you fail to take the sick leave on your time sheet. See http://hr.oregonstate.edu/student-employment for the detailed information in the University wide policy.

NOTE: The sick leave time sheet is only for leave and substitution work. If you are performing hourly work for the department, your hours should go on a separate time sheet that Kelly Carter can set up for you.