So...just don't use mana? Plus they should be generating mana over time naturally to counteract the collar's drain. Or just go to jrpg sleep and refill. Unless the collar also has a prevent regen function.
Either they’re just bad at controlling it or it requires a degree of training not normally given. Hard to tell since we mainly follow Shadow Garden and their members are experts at mana control.
The collar is designed to drain mana and kill, so the cult wouldn't overlook such a design flaw where the wearer can simply regenerate their mana faster than the drain rate (considering they know that Claire and Alexia are among the group). The ghosts also serve as a double measure to make them waste mana since they have to fight without rest.
In a world where humans can do things that cannot be done by humans of this world it is fairly safe to assume that they use magic of some form to do so (in this case mana). Most of the fighting/strength that the people of this world display are beyond what normal humans can do, or at least those of the same physical build.
Basically: physical strengthening and power attack == magic == manga usage. So even without the collar actively ripping manga from them (likely to "feed" or "fuel" something) by fighting (or in some cases simply moving around - i.e. burning manga instead of calories) they are likely expending mana.
But, it's fiction and magic doesn't exist on this Earth, so there's literally no way for me to know for sure any of this.
Separately I have noticed that a few people think that the guy Cid is masquerading around as was that girl's actual brother. She originally said "distant" relative. Page 16, 4th panel, last text box.
Finally, people are questioning the sanity of those staying at the school given the danger, and the sanity of those not freaking out about the danger... Given the general danger of their entire world and that they are there to become "spell swords" and are nobles it isn't all that surprising. FFS they have a magic swordsmanship competition, if they were afraid of getting hurt they wouldn't go to this school in the first place...