Is drop behind a compound word?

[Eso]

English Language Tests, Intermediate level

ESL/EFL Test #156 "Phrasal idioms with the verb 'drop'", question 1

Many parents assume that their children will ......... in school if they spend too much time playing video games.

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

English Language Tests, Intermediate level

ESL/EFL Test #156 "Phrasal idioms with the verb 'drop'", answer 1

Many parents assume that their children will drop behind in school if they spend too much time playing video games.

Correct answer: (d) drop behind
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drop behind is a compound word

[Mister Micawber]

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No, DROP BEHIND is a phrasal idiom.
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Native English teacher at Mister Micawber's