What does this phrasal verb mean: "drop out"?

[Vurhal]

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", question 4

Students who are required to pass a test to enter the 9th grade are more likely to ......... prior to entering 10th grade.

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

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", answer 4

Students who are required to pass a test to enter the 9th grade are more likely to drop out prior to entering 10th grade.

Correct answer: (b) drop out
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I don't quite understand the meaning of the sentence.
Could you please explain it to me?

Vurhal

[Alan]

Hi,

'Drop out' has the sense of leaving a course before you take the final exam. Here the suggestion is that those students who have to take take a test to get into the 9th level/grade will probably leave before they get to the next level.

Alan
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English as a Second Language
You can read my ESL story Well, Hello!

[Lost_Soul]

Hi, Alan

I noticed that you used the article "the" before 9th grade, but left out that article when you wrote 10th grade.
Can I conclude that I can either include "the" or leave it out when I say "8th grade" or "9th grade" ?

Thank you in advance!