Secondary school is usually high school - but some, like the one my kid goes to, is a JR/SR school all in one building.
You only have a month or so to fill out the form.
There is an arbitrary date in October at most schools that you need to turn in the "OPT-OUT" form.
Check with your school for their policy.
If you do NOT fill out the form, then the school is required by law to hand over your private contact info to military recruiters if they ask for it.
Here is a link to a page with a sample opt-out form, just in case your school has not provided one for you.
Here is the OPT-OUT form.