A closed syllable is a syllable that ends with a consonant. The words fan, am, and left have closed syllables. Multisyllabic words have closed syllables too. For example, a two-syllable word with the vowel-consonant-consonant-vowel pattern may have one or two closed syllables.
No, I dont think they should be mandatory, but I do believe that masks should be. Students should get vaccinated to protect others, but at the end of the day it should be their own choice.