Since most employers require experience before hiring system and network administrators, this type of opportunity is a option to get that valuable entry on your resume. The work you will be doing will definitely vary by organization, but most importantly by who you are being managed by. Its ok to ask when presented by an opportunity.
Since you are needing an network administator assistant, a student can be a good choice. It doesnt mean he/she is just a student, no experience, but he/she has the ability to do it(make sure the student is a computer engineering or IT). This is an oppurtunity for the both of you to learn from each other.
yah jorgeM was right it depends on who you are being manage by. best case senario for that is you could grant those task which a network administrator had, which could be a good experience for a student..