goof up vs. goof off

[lost_soul]

English Language Tests, Intermediate level

ESL/EFL Test #193 "Phrasal Verbs (G to H)", question 7

The students ......... when the teacher left the room.

(a) goofed up
(b) goofed off
(c) goofed on

English Language Tests, Intermediate level

ESL/EFL Test #193 "Phrasal Verbs (G to H)", answer 7

The students goofed off when the teacher left the room.

Correct answer: (b) goofed off

Your answer was: incorrect
The students goofed up when the teacher left the room.
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Hi, everybody !

Why not "goof up" ?
(I saw in a dictionary that a man can goof up which means that he takes some drugs. Is it true?)

thanks !

[Mister Micawber]

.
If it is, it is too slangy for me. 'Goof up' more usually means 'make a mistake', which does not apply here; nor do students do drugs when their teacher leaves the classroom-- they do them after school behind the Dempsey Dumpster.
.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber