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If they won't be mentioned again, it's not worth giving them a name - so your first example would be somewhat wasteful. The reader doesn't need to know that the redhead with an aggressive attitude ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/29221 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/29221 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If they won't be mentioned again, it's not worth giving them a name - so your first example would be somewhat wasteful. The reader doesn't need to know that the redhead with an aggressive attitude is called Rob (and maybe it's realistic that your character, Suzy, doesn't even know him by name - exspecially if she is in a large school). Same goes for the blonde. Like the extras in a movie, they need to be there - but sketching just a bunch of details for them is enough. Of course as you realized it's probably better to name them if you plan to make them appear again at a later time. The group of clone-like blondes could be, for instance, interesting to mention again - but that's maybe just because I like the image you used.