Possible Clues:

• Subject of study is awful hat in M&S
• Compulsory British subject
• Algebra, geometry, calculus etc (abbr)
• Trig or calc, to a Brit
• Sums
• Looking back, charlatan wraps summer shirt, as it's integral to game theory
• 18 across subject
• It's about numbers, in short
• For dull, empty heads, a difficult subject, in short
• Scholar takes this after dropping one school subject