خَرَجَ الطُّلَّابُ يُرَاجِعُونَ مَلَاحِظَاتِهِمْ.
The students left while reviewing their notes.
Yurājiʿūna is an affirmative imperfect verb with the plural subject marker. Its clause functions as ḥāl, and its subject refers back to al-ṭullāb. Under the standard rule, an affirmative imperfect circumstantial clause is not directly introduced by wāw al-ḥāl.
In kharaja al-ṭullāb wa-hum yurājiʿūna, however, hum makes the circumstance a nominal clause, which can take a circumstantial wāw. Similar translations do not imply identical syntax.