How hot is elephant toothpaste?
The enthalpy change of the reaction can be calculated using the enthalpies of formation of the compounds in the reaction.  To find the temperature change of the reaction, divide the quantity of heat released by the heat capacity of the solution in which the reaction occurred.

This won't give the exact answer.  Some of the heat will be absorbed by the surroundings including the flask.  The reaction produces gas and some of the energy released by the reaction is used up by the work needed for the gas to be released into the atmosphere.

Rough estimate:

Let's say the teacher used a 500 ml bottle of peroxide.  The solution contains about 0.5 mol of peroxide.  The enthalpy of formation of peroxide is about −190 kJ/mol.  The enthalpy of formation of water is about −240 kJ/mol and that of oxygen is 0.  So, 0.5 mol of reaction gives about 25 kJ of energy released by the reaction.  We'll assume all of it went into the 500 ml of water.  The 500 ml of water has a heat capacity of around 2100 J/K.  So, the solution gets about 12 K hotter.