This is very, very serious, and you should not be posting about it in detail on the internet. This is not a confidential forum. It's information that could be seen by law enforcement.
The impression I get from the tone of your post-- and maybe I'm getting the wrong idea-- is that it hasn't hit home how potentially serious this situation is. You need to seek out the help of a responsible adult who you trust, of course, the first and best option being your parent or a family member. Explain to that person that you NEED-- repeat, need-- to get an experienced juvenile delinquency attorney to talk to. Meet with that attorney and see what needs to be done from there.
There are many people on this website who give free consultations about juvenile delinquency matters (which is an area of law that has a lot of cross over with traditional criminal law, but is not the same)-- myself included. Use that resource to find a lawyer and get connected up immediately.
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I reiterate everything Mr. Brown said about confidentiality. Burglary is entering or remaining upon a premises with the intention of committing larceny or any felony. If you were there to vandalize (potentially a felony) you are guilty of burglary. If you're there to steal, you're guilty of burglary. If you're there to do illegal drugs, that's burglary. Get a good lawyer and consult with him/her in private.
The opinions rendered herein are based on general principles of law. Laws vary from jurisdiction to jurisdiction and there are often numerous factors which can render advice or an opinion inapplicable. You should NOT make any decisions about the handling of a legal matter without first directly consulting with an attorney about the particulars of your case.
I am not sure what the question is, or if you just want comments.
I concur with what prior counsel has said, with a few additional comments. Is the 15 year old in the same school district as the school where the events took place? Is it a public or private school? The depending upon the answers, the matter may be adjudicated through the school district even though the events took place after (or before) school hours. If this is the case, you definitely want to hire a well-recognized, familiar lawyer to the school district as there is a whole new set of rules and regulations dealing with education law that are relevant.
If the events are prosecuted through the juvenile court system, be aware that this offense is a crime of dishonesty so it will have significance to employers and colleges you might want to apply to. The best course may be to immediately hire an attorney to see if an informal resolution with the school can be arranged due to the consequences.