Hi Gloria!
Does your hoped-for restuarant have to be in Argulles? Becuase
Hostal Persal is no where near Arguelles. Arguelles is the district between Plaza de España and El Faro de Moncloa.
I agree with Andrés' recommendation of the Las Bravas chain but I doubt you'd get 26 people inside!! You might get 15 though and I don't recall any of the Las Bravas places having any table-seating, only a few bar stools but mostly standing at the wall-counters or at the bar. I LOVE Las Bravas though. GREAT food, really but I wouldn't call it a "restaurant" though.
You're looking for an ACTUAL place called "Casa de Tortillas" or some place which specializes in tortillas de patatas? Most EVERY restaurant and tapas place sells tortilla de patatas (a.k.a. tortilla española) and they're almost always very good.
I would HAVE to fully, whole-heartedly recommend the LARGE El Brillante. But like Las Bravas, it's not a restaurant and don't think they have anything but standing room at the bar/walls and I doubt the terraza outside on the sidewalk will be open yet in March but maybe! Anyway, they're good and cheap and popular with madrileños just as are Las Bravas.
As you mentioned, Museo del Jamon does have seating and they're cheap too but you can't eat there for every meal. Touring with large groups like this is ALWAYS a real logistical "challenge" (I was going to say "nightmare" but wanted to be more positive sounding. heheehe..). I know we have a few educators here who have taken school groups to Spain and maybe they can help out.
Antonio/Andrés, can you suggest some place large enough, and more-or-less cheap, which can accomodate 26 people WITH seating?????!!?
I'd think this would be tough. You might have to split up into 3 or 4 groups.
Good luck! Saludos, MadridMan