“Number” vs. “Amount” and “Fewer” vs. “Less”: Use "number" and "fewer" when you are referring to things that can be counted individually like "students" and "books." Use "amount" and "less" when you are referring to things that can’t be counted one-by-one like "time" and "money." NOTE that you if you counted increments (things that can be counted one-by-one) of time and money , you would use different words like, "hours," "minutes," "dollars," "coins," etc.
Example: "Each classroom has the same amount /number of students." The correct answer is "number" since "students" can be counted individually.
Example: "I asked the waiter for less/fewer ice in my drink." The correct answer is "less" since "ice"cannot be counted individually. NOTE: if the sentence read, "I asked the waiter for less/fewer ice cubes in my drink," the correct answer would be "fewer" since "ice cubes" can be counted individually.
Now, go TEACH it!