The piece of Play-Doh(TM) Jesse pulls out of the calculator changes shape slightly between shots.
The Play-Doh is apparently what caused the calculator to give wrong numbers, but putting Play-Doh in a calculator would simply cause the calculator to stop working entirely.
When the students come in and begin chanting R.E.M.'s name, it cuts to a close-up of Joey and Steve and two students standing behind them who are chanting out of sync.
None of the girls from REM are playing their guitars.