Earlier quoted context omitted.
fyi these kinds of pen cases have been popular in asia and asian communities worldwide for decades, usually for carrying mechanical pencils and their accessories (extra lead, erasers, protractors, rulers, mini notebooks, sticky pads, whatever) western equivalent would be trapper keepers with their pencil pouches.
You can also probably get little pencil boxes at an art store or some place like that. Would recommend. A little plastic box that prevents me from having to fish around in my backpack possible get poked when I want my pencil is great, and they are pretty cheap.
Compasses, dividers, protractor, setsquares (that we don't use these days) and a 15cm ruler. Pencils, sharpener, rubber. Usually a stencil for lettering and a leaflet with geometry definitions.
Available from supermarkets, stationers and cash and carrys right now because it is GCSE Maths exam time.