To be able to contribute to the scheme you have to be employed as a teacher. Supply staff employed by private companies do not fall within this definition. If you are employed by a school or local authority and work regularly, particularly in one school, ask if your service can be pensionable. If not, make other arrangements, such as a stakeholder pension. When you return to regular service, you can start contributing.
Then ask about adding other contributions to your record with the scheme.
Remember, you are likely to be of an age where you will have to work to 65 before claiming a full pension.