Instead of telling concerned people "not to fly", the flight school should have established cleaning and disinfection protocols before reopening. And those could have included substances which are appropriate for the purpose and would not damage the aircraft. Airline aircraft also have their cockpits disinfected frequently these days - and I haven't seen any ruined panels as a result of this.
Of course, if there's such a protocol in place, it has been made known to everyone but nonetheless someone knowingly deviated from it - this is entirely their fault and they should be the one to pay for restoration.