May I know if membership in the Girl Scout Movement (Philippines) is mandated by law? What if we won’t join?

Highly unlikely that membership in the Girl Scouts is required, as it is a voluntary organization. I am not an expert on Phillipino law, so the best thing is for you to contact a school teacher or trusted clergy person and ask.

If, by some reason, membership were mandated by law, and you refused to join, the law would spell out the consequences. It may be a fine, public service, or loss of privelege.

Again, highly unlikely that it is a law, but check with an authority in that area.