I cannot address the school issue. However, as far as the marijuana charge is concerned, no amount is acceptable. It does not matter if it was not "usable". If that were the case all someone would have to do to avoid a possession charge is make sure that he/she smoked most of it before getting caught.
As for the Miranda issue, the minor was entitled to be read his rights under the circumstances you described. As a result, an statements he made without waiving those rights or being allowed to contact a parent or guardian would be be inadmissible.
You should not post any more details on this public forum. Most defense attorneys offer a free consultation. I would recommend consulting with and hiring a local defense attorney.
Your questions are too complex to be answered in this forum. You need to immediately speak to a lawyer who handles juvenile court matters in Houston as well as one who handles educational matters such as school expulsions. If you can't afford a lawyer for your child apply for a public defender immediately. You can also inquire at the county bar association to see if there is a reduced fee or pro bono program to answer your questions about the school matter.
I agree with Mr. Patterson here. Juvenile law is a very specific area and a complex mix of criminal and family law. The questioning of Juveniles is not like adults. There are many protections and exceptions under Texas law. You need to speak to a criminal defense attorney that does juvenile law. If you can't afford a lawyer you can apply for counsel for your child at the time the case cycles up to court as all these cases do now a days.