There are basically two yeshiva options in Israel: Hesder and non-Hesder.
Hesder yeshivot are populated by yeshiva men. Non-hesder yeshivot are populated by yeshiva boys.
The age range in both is the same.
Back in the day, almost all "Israelite" men between 20 and either 40 or 60 went to a discretionary war. There were four (4) exceptions:
1 The person is newly wed.
2 The person who just acquired a new house and has yet to dedicate it.
3 The person who planted a vineyard and who has yet to taste its fruit.
4 The person is a coward whose presence would demoralize his fellows.
Most of the yeshiva boys are not newlyweds or new home owners, or planters of vineyards.
In any event, the exemptions were only for a "discretionary" war (vs. an "obligatory" war). In an "obligatory war," "all go forth, even the bridegroom out of his chamber and the bride from her bridal pavilion" (Mish., Sot. 8:8).
Bottom line: There was no special exemption of yeshiva boys.
Back in the day, the leadership - Pinchas and Joshua - led the warriors into battle.
There was a somewhat heated discussion between Moses and the leaders of the tribes of Gad and Reuben who wanted to stay on the east side of the Jordan. In the end, they got to stay put ON THE CONDITION that they would aid their brothers conquer Canaan.
An obligatory war is defined by Rambam as a war like that fought by Joshua to liberate the land of Israel from the Seven Nations, the war to eradicate Amalek, or a war "to defend Israel against an enemy that attacks them.". The latter pretty much defines Hamas, Hezbollah, ISIS (or whatever the nom du jour), and similar groups bent on testing HaShem's will.
A compromise for yeshiva boys
The haredim who refuse to defend the country that defends them, in lieu of military or national service, should be sent in caravans (trailers) to the front with the troops. They could stay inside the trailers - clearly marked as yeshivot - and study the talmuds and recite psalms for the young men and women with the guns. Surely all of Israel's enemies would avoid placing the yeshiva trailers in danger.
Better, the yeshiva boys could do non-combatant tasks. Secretarial jobs, cooking, kashrut supervision, medic and litter bearer functions and ambulance drivers. Maybe they could be issued uniforms with unique markings to identify them as non-combatants so the enemy will not attack them. (Attacking non-combatants is a war crime, and Israel's foes would never do that.)
I think HaShem was wise to include the cowards' exemption; in fact, I think it should include the obligatory war as well as the discretionary war. Cowards are demoralizing; cowardice is contagious. Since the yeshiva boys apparently are cowards, don't draft them into the army, but do present the option to study the talmuds in the comfort of a trailer at the front or performing non-combatant work to free up braver men and women to defend their country.
Any yeshiva boy who refuses loses all future stipends from the state - for life - and the yeshiva in which he studies has its government subsidies and tax breaks reduced. (I'm sure the yeshivot will survive on funds begged from the U.S. and Europe.)
Much of the foregoing is from The Jewish Virtual Library, https://www.biblegateway.com/passage/?search=Numbers+32&version=ERV. The page considers a number of war-related questions.
