Assuming the fire is caused by an electrical problem, you would need a class c extinguisher.
Ordinary burning solids (wood, cloth) would need a water extinguisher (Class A). Burning metallic solids (lithium, magnesium) would need a special power found in Class D fire extinguishers (in the USA). A Class ABC extinguisher would also work on ordinary combustibles (Class A fire), but not on Class D solid fires.
A classroom would generally contain only "ordinary combustibles", so a type A fire extinguisher would be appropriate. In a chemistry laboratory, cooking class, automobile engines, electrical or welding class, different rules would dictate the type, placement and number of required fire extinguishers, typically Type ABC (multi-class).
You need to use a Class B extinguisher on flammable liquids.
A water fire extinguisher (called a "Class A"extinguisher) is only useful on fires involving wood, paper, cardboard, and most plastics. An extinguisher labelled "Class B" is for fires involving flammable liquids, and "Class C" is for electrical equipment fires.
Foam, dry powder or CO2 gas- a Class B extinguisher
Trick question, if the fire extinguisher is full of water, such as a Class 2A. However, a fire extinguisher is designed to put out a small fire quicker than anyone can respond with a hose and a water supply. Unfortunately, many fires react violently when water is applied to them, so a different type of fire extinguisher must be used.
class c fire extinguisher
A Class B fire extinguisher. Below is an article about the different types of fire extinguishers.
B-I
It depends upon the type of chemical that is burning, but frequently it is safe to use a dry-chemical powder (DCP), or an ABC type of extinguisher. However, boiling grease fires may need a Class K extinguisher and flammable metals (aluminum, magnesium, lithium, etc) may need a Class D extinguisher.
Water in a very fine mist from above, as only professional firefighters are able to do, will help cool the fire, but 'hosing' it will only spread the fire