How did you find out "they were pressing charges?" From the school directly, or did you receive something in the mail?
If you are lucky, the Maricopa County Attorney's Office may offer you the chance at pre-indictment diversion (also known as TASC, which stands for Treatment Assessment Screening Center). TASC would require you to drop clean UA's over a period of time, after which, the case would be dismissed. Who gets a shot at this is 100% up to the County Attorney's Office, so watch your mailbox.
If you DON'T get a pre-indictment TASC offer, you will most likely receive a Summons, sent certified mail, compelling you to appear before the Maricopa County Superior Court to face the felony charges of Possession of Marijuana and Paraphernalia. The good news is that you will most likely receive a TASC offer at this stage as well, provided you don't have a previous criminal history.
Lastly, you could be charged with misdemeanor Possession of Drug Paraphernalia, if there wasn't a "useable" amount of marijuana present. If that's the case, you will again receive a Summons, but it will be to either a Municipal/City Court or a Justice Court. TASC will most likely NOT be available, but, depending on the Court, there may be other diversion-type options for you that ultimately won't leave you with a conviction.
If you are in Superior Court and you cannot afford an attorney, one will be appointed for you by the Court, free of charge. If you are in Municipal/Justice Court and the State is NOT seeking jail or probation, you will be completely on your own.
You can ask that the court appoint an attorney but if you are not looking at jail time, one will not be appointed. You should plead not guilty at the first hearing and see if the prosecutor will offer you diversion or let you plead to possession of paraphernalia.
This answer is intended to only provide general information and does not establish an attorney- client relationship. The information is not guaranteed as sound legal advice since most questions are very limited in nature. You may wish to consult with an attorney regarding your specific case.