Collective nouns are the names of collections of things, or, more usually, people, for example 'a shoal of fish', 'a herd of sheep', 'a team of students' and 'a platoon of soldiers'. In the strict grammar of academic writing, a collective noun is singular, although it denotes a (plural) number of things.
There is a grammatical issue here. Pedants sometimes complain about errors connected with number, and often about such problems in connection with collective nouns. This is an area where psychology and grammar arrive at different answers. The reason for most of the errors that are seen by grammarians is the way that the individual writers perceive things. A committee contains several people, but is best thought of as a unit. The Cabinet is made up of some 20 Ministers, but is a single body. Ideally you should use a singular verb with a collective noun - "the Government thinks that" -- but many writers do not follow this rule. They have some justification when they are thinking about the behaviour of individuals - e.g. "The Committee have expressed many opinions" (rather than has), or "The Cabinet disagree about this" - though here it is better to say "Members of the Cabinet disagree".
As always, good writers think about their readers. In the case of students, this means observing the tastes of, and rulings made by, their teachers. If your teacher is concerned about the agreement of number, then be careful, and follow the guidance in the third paragraph on this page. Always try to think logically, and use collective nouns with care.
There are, however, three points which even the least pedantic teacher will insist on:
- If you use a plural verb with a collective noun, you must use the plural forms of the relevant personal pronouns and possessive adjectives (e.g., 'The team have lost their last three matches'), while if you use a singular verb, you must use the singular forms ('The team has lost its last three matches'). I.e., what you cannot say is 'The team have lost its last three matches'.
- If a collective noun is preceded by the indefinite article ('a' or 'an'), it must have a singular verb. While you may say either 'The firm is treating its employees unfairly' or 'The firm are treating their employees unfairly', you must say 'A firm which treats its employees unfairly will not keep them long' and cannot say 'A firm which treat their employees unfairly will not keep them long'.
- It may console you to know that many famous writers have used collective nouns loosely - 'wrongly', in the pedantic sense. These include Thomas Gray, whose much anthologised Elegy written in a Country Churchyard contains the line "The lowing herd wind slowly o'er the lea". Here the collective noun 'herd' [= 'a collection of cows'] is used with a plural verb 'wind', where a pedant would insist on 'winds' - or a different noun: "The lowing cows wind slowly o'er the lea." But that might not sound so good.