Simple. Describe your motive to administrative that what your intentions are and what do you want to do on Administrative account which you couldn't do otherwise? He/She might let you use account on his/her presense.
In most cases administrator will refuse the access and probably will ban you using any computer in the lab but still it all depends what is your motive? Is it educational related to your class assignment, in this case he/she will let you use their account in his/her presense, if it is not don't even bother to ask to use administrative services because you can get into trouble.
Peace