Now You See Him, Now You Don't
(1972)

College student Dexter Riley (Kurt Russell) invents a spray that turns things invisible, and washes off with water.  Mr Arno, the local mob boss, pretends that he wants to help the colleges financial problems by buying the mortgage to the college's land.  Dexter and Skyler make themselves invisible to enter Arno's office. They find models proving that Arno's real plan is to tear down the college and build a huge casino.

Arno finds out they have this invisibility formula and sends one of his men to steal it.  They substitute a fake spray can.  When Dexter presents his invention at the scientific conference, he is embarrassed as the fake spray does not work.  The students plant a bug in Arno's office and learn that Arno stole the formula, and Arno is planning on turning invisible to rob the bank.

The student try to stop the bank robbery, but are too late.  They pursue Arno and run his car off the road into a pool.  The water turns Arno and the stolen money visible.  Dexter retrieves the true formula in time to rush back to the science conference.  Dexter wins the conference and the award, saving Medfield college again.  

Thanks Joseph C!