First, you may want to check that the school has followed their own policies and procedures. You have the right to ask to see these policies and procedures.
For example, for all government schools, the documents ‘Effective Schools Are Engaging Schools’ sets out guidelines about suspension and expulsion. You can find this document online. Also, each university or other educational institution will have their own policies and procedures and you need to get these and read them.
You might find that these guidelines have not been followed. You can use this to stand up for your rights.
Second, if your expulsion or suspension is to do with your disability, you should get help from an advocate or lawyer to find out whether the suspension or expulsion have broken any other laws. For example, depending on the situation is might have been against laws like the Equal Opportunity Act or Disability Discrimination Act.
Depending on what you find out from all of this, you can then write to the education provider, challenging your expulsion or suspension. The same goes for being told you can’t attend an activity such as a camp or excursion.