A pair of scissors is or are? which one? My teacher at lyceum said that it is are but I want to make sure.
However when you say "a pair of scissors", you are referring to one pair, hence it's singular.
A 'pair' is two of something, but a pair can be singular or plural—it’s one of those odd English nouns (like couple) that can be singular or plural depending on how you’re thinking of the people or items in question.
The noun 'pair' can be followed by a singular or plural verb. The singular is always used when pair denotes the set taken as a single entity: This pair of shoes is on sale. A plural verb is used when the members are considered as individuals: The pair are working more harmoniously now. After a number other than one, pair itself can be either singular or plural, but the plural is now more common: I bought six pairs (or pair) of shoes. (Source)
Since 'pair' is a single entity and is a collective noun, you should use 'is', for your context.