1) "The hesitant student comes up first, releasing a howl reminiscent of a wounded animal."
2) "...until a dust cloud appears on the horizon, approaching fast."
Also with sentence #2...should "quickly" be used instead of "fast" or should it be "fast approaching" or is it okay the way it is? Thanks.
I would use quickly.
I think you need the comma in your sentences. There the present participle construct is used to give additional information about the subject, and the rule is to put a comma in such a case. Otherwise, some confusion could take place. For example, let's compare the two sentences below.
 Bill gazed at Monica, talking on the phone.
 Bill gazed at Monica talking on the phone.
In #1, the person who was talking on the phone was Bill, but in #2 it was Monica.
As to the modifier for 'approach', I have learned, in general, 'quickly' is used to state about an instantaneous action, whereas 'fast' is used to talk about the speed of a durative activity. So my choice is rather 'fast', but I might be wrong.
People are waiting to help.
Related forum topics: