Incorrect: In our school the number of students is less.
Correct: In our school the number of students is small.

Explanation

Less is the comparative of little. Comparative forms are not used in sentences where no comparison is implied. But is the sentence ‘In our school the number of students is little’ correct? No. The adjective little can be used only in the attributive position (before a noun). In the predicative position (after a verb like is) we have to use a word with a similar meaning.

Incorrect: From the two she is pretty.
Correct: She is the prettier of the two.
Incorrect: Of the two routes this is the short.
Correct: Of the two routes this is the shorter.

Explanation

When a comparison is made between two people or things we use a structure with of, not from. Note that we use an adjective or adverb in the comparative form to compare two people or things.

Incorrect: From the three he is the smarter.
Correct: He is the smartest of the three.

Explanation

To compare more than two people or things we use an adjective or adverb in the superlative degree.

Incorrect: There is a best student in that class.
Correct: There is a very good student in that class.

Explanation

It is wrong to use comparative and superlative forms when no comparison is implied.

Compare:
Charles is the smartest boy in the class. (Here Charles is being compared with other boys in the class. Therefore, we use a superlative adjective.)
He is the smarter of the two brothers. (Here a comparison is made between two people. Therefore we use a comparative adjective.)
He is a smart boy. OR He is very smart. (Here no comparison is implied. Therefore we use a positive adjective.)

Incorrect: I have never seen a so good boy.
Correct: I have never seen such a good boy.
Correct: I have never seen so good a boy.
Incorrect: He was a so big man that he could not sit in that chair.
Correct: He was so big a man that he could not sit in that chair.

Explanation

So is very often used in the rather formal structure so + adjective + a/an + singular countable noun. Note that it is wrong to put the article before so in this structure.

Incorrect: Yours affectionate brother
Correct: Your affectionate brother
Correct: Yours affectionately
Incorrect: Your lovely friend
Correct: Your loving friend

Lovely doesn’t mean the same as loving.