This question in itself is very much suspicious because i would Wonder why you would want to hack into someones account so you can get to change their balance and do whatever it is you wish to do with the said account without been detected just like you asked, it's really sounding like an unsafe intention despite the fact that it's looking like just a question but then what you did asked about just so you know is actually possible.

There have been times where we got to hear of sites been hacked by a set of hackers and  good amount of funds lost to the incident so i wouldn't want to think it's not possible to get hacked and account balance been changed by the hackers but then it's most definitely not going to be an easy task except for site built by students as you did mentioned and they happen not to be professional enough then it could be assume the firewalls my be weak and easy to breakthrough and gain access to the admin area and possible manipulations done .

These sites usually created by these young and yet to be professional sites are usually not used most times for professional sites but maybe for test running of projects and other possible reason but due to the probability of been possibly hacked, it's usually not been used so as to avoid future problems. I have been wondering if this post is even supposed to be in this thread, that should be looked into and moved if need be otherwise. It can be allowed here.