Comparatives and superlatives cannot be used in sentences where no comparison is made.
In our school the number of students is smaller.
(Smaller cannot be used because no comparison is made.)
- In our school the number of students is small.
(Small can be used because it is neither a comparative nor a superlative.)
- Jimmy is the best singer in the group.
(Best can be used because Jimmy is being compared with the other singers in the group.)