You would be able to help much, much, more by being a Governor. Really you would. It could help the whole school in a very meaningful way. This is because you will be engaged with recruiting more pupils, making the school attractive to parents, recruiting the right staff (difficult, but staff want to work in a well run school), improving SLT by performance management, setting targets and strategic aims, and you can engage with Maths/Science and be the Science or Maths Governor. They would welcome you in this role.
I think going into a secondary school and helping in a practical way is fairly unusual and makes more work for the staff quite frankly. As a Governor you can get involved in school trips, accompany university taster courses for science - there are endless things you can do as mentioned above by Rufus but being a Governor gives you more clout to get things going. You are not just an agitating parent!
You would also have a great handle on the progress the children are making and be aware if lack of good maths and science teaching is holding the children back. You, within the umbrella of the Governing Body, and the SLT would be responsible for improvements. There is no better role than this.
All schools are short of money and you cannot squeeze a quart out of pint pot so Governors have to make sure as many parents as possible want to come to their school to raise income by higher numbers on roll. PTA money in small, standard catchment, schools never amounts to much in overall terms. Small and cosy just does not work in the secondary world because the budget will always be tight if there are not enough children and too many needy children. You need a healthy school number and a good balance of children.
You really can help, but in a professional way on the GB, not just a concerned mum who wants to help out.