As for wearing "appropriate footwear" the instructor was doing his job correctly and probably correct in advising you NOT to drive in certain footwear!
I have held an instructor's licence for 31 years and I will always advise novice students that it's unwise to wear thick or clumsy footwear, paticularly when it's their first lesson - in fact anyone booking with me is told prior to the first lesson to try and provide something akin to deck shoes. If not, it's sometimes difficult to tell whether they are having problems in not understanding how to control the clutch or the problem is the lack of feel due to inappropriate shoes which makes their learning more difficult.
For some people, wearing inappropriate footwear when learning to drive can undoubtedly add several hours of extra tuition to their training to gain smooth control over the clutch which comes at a cost to the student and only a questionable instructor would not offer that advice.