Having some difficulty reconciling in-game plot logic with the metagame

I'm trying to figure out a situation where in-game logic and metagame logic are at odds with one another.

As the first act of an adventure I'm preparing, the PCs show up to an open call for recruits by an organization of forest wardens looking for new recruits to help patrol the land around a city. They need to demonstrate their skills to the organization in order to gain probationary membership, after which they get assigned a proving mission (i.e. acts 2 and 3).

The open call is necessary because I'm going to be running this at conventions for multiple groups of strangers, few of whom are likely to know one another in advance. The recruiting call is designed to give the players an opportunity to introduce their PCs, demonstrate the PCs' abilities, and give the PCs a reason to work together (i.e. they're all new recruits of the same organization). Most will be using pregen PCs that I supply, but a few may take the time to build a PC in advance and bring it.

The problem is that the things PCs can do to demonstrate their skills -- sparring, athletic contests, skill checks, and so on -- involve die rolls. And once you get the dice involved, there's a risk that one of the players will just roll terribly and fumble their trial, which would put me in the awkward position of having to explain why this one character who was so obviously terrible at what they do would get recruited anyway. I hate the prospect of running roughshod over in-game logic for metagame concerns, but I can't just dismiss a player from the table because their PC failed the recruitment tests due to poor rolls.

The organization wants competent people, and need to have some way of picking those out. But from a metagame perspective, the PCs have to succeed or there's no adventure.

I've been struggling to come up with a way of reconciling these. Any suggestions? The best I've got is that in the event of a failed test, one of the judges (who are higher-level members of the organization) sees something promising in a candidate whom everyone else has written off, and insists on their recruitment anyway. But it's kind of an obvious deus ex machina solution.