If you were on probation from stealing from your job then your new cases would violate the terms of your supervision (not to be rearrested).
On the new cases you would be facing a maximum of 5 years in prison jon each of the grand theft cases (assuming the value was between $300-$5000). The potential penalties get much more serious if you exceed the $5000 which then would be 15 years in prison.
On the petit theft charges you would be facing 60 days in jail on the first one and up to one year in jail on the second one.
Being an 18 year old student does not mean you will not do any time at all. However, a very structured term of monitored supervision is not out of the question, depending on the Judge and the prosecutor you have.
Speak to an attorney about your options. You will likely not score mandatory prison time on these types of offenses, but judges are often angry when people steal in positions of trust. The strengths of your defenses (and the difficulty of proving the case) may allow your attorney to negotiate a favorable deal for you.