What's the point of letting the Forsaken be?

[Zuleika-hyjal (Zuleika)]

I double-checked the quest text, and I disagree that it’s “clear there were many students left in the school.” I’ll put the text of the quest chain below.

https://www.wowhead.com/quest=25880/warn-master-thaldarah

There are dozens of students at Thal’darah Grove. Most of them have no means of defending themselves, ripe for the slaughter.

You must get word to Master Thal’darah of the Horde’s plans. He will direct you in what you must do next.

Fiora Moonsoar has granted you the use of her fastest hippogryph. Take it now to Thal’darah Overlook! Don’t look back, (name)! The Horde must be stopped!

https://www.wowhead.com/quest=25889/save-the-children

My poor, defenseless young students are stranded at Thal’darah Grove. With the battle raging across Battlescar Valley, they have no means of escaping. We must save the children before the Horde drops their bomb!

Take my hippogryph and use it to rescue the young druids of Thal’darah Grove. Drop them off at the hippogryph post here in town.

Save them all, (name)! You truly are our only hope.

https://www.wowhead.com/quest=25891/last-ditch-effort

The Horde have signaled a retreat. Look now, they fall back from the field of battle - portends of disaster.

We must make one final descent into Thal’darah Grove. I could not live with myself if any of our young druids were left behind.

Assist me, hero. I will take the reins. You search for any druids that might be left behind.

We will go when you are ready.

The fact that you rescue five “helpless young druids” for the second quest is obviously just for gameplay—they wouldn’t require each PC to pick up “dozens” of students, so either each one stands in for a larger group, like when you see armies onscreen, or you assume that multiple PCs are doing the quest at the same time and each one is picking up a few.

And if they meant it to be clear that you definitely didn’t get all the students out, I don’t believe they would have worded the last quest as they did. They would have Master Thal’darah say “More students are still in the school and we must rescue them all before time runs out!” rather than “Search for any druids that might be left behind.”

Then the cutscene contains this dialogue:

Look there! The Krom’gar balloon is heading towards the grove! We must hurry!

(bomb explodes)

We’re too late. The grove is destroyed and with it our hopes and dreams. The Horde will pay for this atrocity! Hellscream must answer for this crime.

Again, Master Thal’darah just says “The grove is destroyed,” not “OMG, those jerks killed my students!”

I would say it is ambiguous at best. There may have been a few stragglers that the Alliance PC doesn’t succeed in rescuing, but there is no firm evidence that this is the case.

EDIT: Video of the entire quest chain in case you want to see for yourself:

World of Warcraft: Cataclysm, The Bombing of Thal'darah Grove, Stonetalon Mountains Highlight