If I am correct, Woodward is a private school, but you do not specify whether the school which expelled you is also a private school. If it is a private school, you have far fewer rights and I suspect that your presence on the grounds would cause that school to call the police. If the expelling is a public school, I wonder whether you exercised or were told about your rights to appeal an expulsion. If you were on an IEP, you would have even more rights.
If this incident happened not in the recent past and you do not have a "No Trespass" order that has been served on you, you might chance attending outdoor events like soccer, especially if you keep a low profile. Indoor events might be more tricky as your presence might be noticed. If there is no "No Trespass" order in place, the public school might ask you to leave or might call the police to escort you off the grounds. You should not be charged with a trespass violation, but if the police escort you off grounds, I think you will soon be served with a "No Trespass" order which if violated could result in criminal charges.
You should consider the amount of time that has transpired since your expulsion, as well as how important it is for you to attend these events. You should consider also requesting permission from the expelling school to attend these events, only.
I regret your expulsion especially since marijuana possession of less than one ounce is a civil, not criminal violation and schools should not be expelling children for this violation. The law specifies no negative consequences under other laws, for small amounts possession. This is true despite the opinion of the Commissioner of the Department of Elementary and Secondary Education.
This is not a legal issues.
Send a nice respectful letter to the principal at the former school expressing your remorse for the conduct that got you expelled and acknowledging that you do not have a "right" to attend events there, but asking with respect if s/he would be willing to allow you to attend events there on condition that you pledge to be follow ALL the rules and regulations of appropriate behavior. This is an opportunity for persuasion.
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No you may not go back. You should ask your parents to read the letter that they or you received explaining the expulsion. The fact that you even asked the question shows that you are thinking about your actions. Woodard is the public school in Worcester that Worcester sends it's expelled students to - it's not a private school. (To clarify the other answer).
You are not precluded from sending the superintendent a letter requesting readmission at some point - I am assuming you didn't appeal the expulsion during the ten day time frame that is provided by law. If you can show that you have attended some type of drug rehabilitation program and are clean you may have a chance for next year. Good luck.