#anyone left at the monastery has plausible deniability

[iturbide]

Tbh tho I can't help but be curious - what's the game plan when everyone gets to Fhirdiad? Are there are stragglers (either showing up late or not showing up at all)

Well the whole purpose of heading to Fhirdiad in the first place is for asylum! Since Rhea is going to figure out that Byleth's missing from the monastery, they're all certain that she's going to send the Knights of Seiros out looking for them to bring them back and pull off whatever her ritual at the Holy Tomb is. With that in mind, they need to put as much distance between themselves and Garreg Mach as possible, and ideally set a course for somewhere safe.

That last condition is the first major obstacle. While the Empire might seem like a good choice on paper, given the Church's lack of influence there, Edelgard is still just the princess rather than the empress. While Edelgard might not have had trouble going home under the nobles' radar with just Byleth, Hubert, and a small guard, there's no possible way to pull something like that off with a bunch of her classmates in tow. The Alliance also has its benefits, since the Church's influence is generally weak there -- but Claude has even less potential power than Edelgard, given how the Alliance is structured, and there's no guarantee that the round table might not elect to hand them over to Rhea anyway.

This leaves the Kingdom -- and while the Church has a much stronger foothold there than elsewhere, Dimitri is also 18 and legally eligible to ascend the throne by the time of their escape from Garreg Mach. If he does become King of Faerghus, he would have the legal authority to grant Byleth and the other students asylum, and while Rhea is certainly scary and pretty desperate at that point, she's not crazy, and no one believes that she would attack a sovereign nation that the church itself had endorsed just to retrieve Byleth -- who, again, would very publicly be an asylum-seeker rather than a hostage. This makes Fhirdiad the ideal destination post-escape.

As for whether there are stragglers, there really aren't! Before leaving the monastery, Byleth took care to warn the students from all three classes that something was going on and that they were leaving, without ever saying where (in part because they didn't know at the time) -- what they didn't expect was the outpouring of support from the students, with several from each class insisting on joining them to help keep them safe and uncover the truth of what's going on in Fodlan. They are all explicitly cautioned to think hard about whether they really want to come or not, and if they decide to join, they have a deadline to pack and join the group leaving the monastery: anyone who isn't part of that initial escape would have no reasonable chance of joining up with the group again, since it's only after getting out of the monastery and making a solid half-day's progress away from the monastery that they decide on their final destination.

From that point on, the group also sticks together, keeping a 'no man left behind' mentality (because honestly, ending up alone in Faerghus, especially for someone who isn't equipped for the conditions, is a death sentence). No one is separated from the group for longer than a few hours because every effort is made to find those who get lost, up to and including a mandated 'buddy system.'

(The one exception to this is Flayn, who does not come with them from the monastery because Edelgard doesn't tell her about the situation -- but that's where the comedy of errors comes in.)