To figure out whether the quantifier is singular or plural, we need to check one more thing
Sometimes, a quantifier refers only to one thing, not many things.

Example:

Each, every, and one always refer to one thing, but 10%, half, all, and most would refer to more than one thing if the object of the preposition is count (with one possible exception that I will show you in a second).

Of course, if the quantifier is always singular, then the verb must always be singular, too. (Let's not forget our common sense in grammar, okay??)

Example:

One of the people is...
Each of the students is...

Conjunction and Verb

The teacher together with the student IS (or ARE) going to...?
The teacher and the student ARE (or IS)going to?

Generally speaking, we need a conjunction to create a plural subject from more than one singular noun. "together with" is NOT a conjunction, and therefore cannot create a plural subject. "and" on the other hand, IS a conjunction and CAN create a plural subject.

"A number of ..." always takes plural verbs.
"The number of ..." always takes singular verbs.

Example:

The number of people has increased
A number of people have gone

The important thing here is that the number in the first example (the number of women employed outside the home) is an actual number--35,000, for example. Even if you add more women to the original number, there will still be one number, right?

The second usage of "numbers" is also correct, and means that there are many people in that group.

Example:

People are leaving California in greater numbers.
People are spending more money on the Internet in greater numbers.

Second, "curfew" is a singular count noun and therefore requires a determiner (the).
I agree with you that skill can be both a count noun as well as a non-count noun. It all depends upon the context.

Have a look at the example below:

1. Harry knows quite a few driving skills.

Conversely, if were to ask Harry about his driving skills, I would ask.

2. How much skill do you have in driving a car, Harry?

So you see, the word "skill" remains the same but depending on the context, skill can be a non-count or a count noun?

With fractions, percentages and indefinite quantifiers, the verb agrees with the preceding noun or clause. With singular or non-count nouns or clauses, use a singular verb:

Example:

One third of this article is taken up with statistical analysis.
All of the book seems relevant to this study.
Half of what he writes is undocumented.
About fifty percent of the job is routine.
All the information is current.

With plural nouns, use plural verbs:

Example:

One third of the students have graduate degrees.
Fifty percent of the computers have CD-ROM drives.
Many researchers depend on grants from industry.

With collective nouns, use either singular or plural, depending on whether you want to emphasize the single group or its individual members:

Example:

Half of my family lives/live in Canada.
All of the class is/are here.
Ten percent of the population is/are bilingual.


Recommended Books

1) Sentence Correction GMAT Preparation Guide, 4th Edition (Manhattan GMAT Preparation Guides)

2) The PowerScore GMAT Sentence Correction Bible

3) Kaplan GMAT Verbal Workbook