Can I cancel an agreement with a school?

When my daughter started secondary school, I signed a parent teacher agreement that I would work in partnership with the school towards my daughters education.

This agreement points to the school policies, and, although all the policies are not written on the agreement, the document basically states that my daughter agrees to abide by these policies, and that I, in a round-about way agree to the school enforcing those policies (ie – detentions after school and detentions at dinnertime). My daughter also signs as does a representative of the school.

Anyway, I’m currently in a bit of a wrangle with the school because they informed me via text, 1 day before an after school detention, that they would be detaining my daughter in detention.

1. I didn’t receive the text (they say they sent it)
2. I’ve informed the school that my daughter is not to attend a detention without my express permission.
3. My daughter (naughty naughty) didn’t inform me about the impending detention the night before.

To cut a long story short, I had a lengthy conversation with the school, and basically informed them that in future they need my express permission before detaining my daughter. They said they didn’t need to get my permission for dinnertime detentions and pointed out that I’d signed the agreement mentioned above.

So, my question.

If I inform the school I no longer agree, is the agreement dissolved / null & void?

My assumption is:
1. That an agreement is not a contract.
2. that I can ‘step out’ of the agreement if I so wish.
3. Even if it is a contract, there is no time limit specified, and no mention of any penalties for breaching or ‘stepping out’ of it.

All opinions welcome as are legal definitions etc.

Lastly, although I cut the story short, I informed the school that in future, permission asked for, that I would have no problems with detentions, provided I thought they were justified. I informed them that I had dealt with the situation myself by grounding my daughter for hiding the detention, and leading me into a conflict with the school, and also made her write a written apology to the teacher for disrupting the class.

The school have informed me that they don’t really agree with my wishes, and are wanting my daughter to finish the detention tomorrow dinnertime, I said not without my express permission and I’m not giving it out of principle, and in any case, I’ve already dealt with the matter.