What is the difference between drop off and drop out?

[Dropper]

English Language Tests, Intermediate level

ESL/EFL Test #156 "Phrasal idioms with the verb 'drop'", question 2

Children who ......... of school face a high risk of sexual exploitation, hazardous labor, and living in the street.

(a) drop off
(b) drop out
(c) drop behind
(d) drop in

English Language Tests, Intermediate level

ESL/EFL Test #156 "Phrasal idioms with the verb 'drop'", answer 2

Children who drop out of school face a high risk of sexual exploitation, hazardous labor, and living in the street.

Correct answer: (b) drop out
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childrenwho drop off of school face a hihg risk of sexual explotation, hazardous labor, and living in the street.

[Mister Micawber]

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The correct answer is 'drop out' (= stop attending), not 'drop off'.
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Native English teacher at Mister Micawber's