Why class is the hardest place to do nothing
At a training class, doing nothing is surrounded by everything your dog finds interesting. Other dogs are moving through exercises a few metres away, treats are being delivered constantly, instructors are giving cues in excited voices, and your dog has learned that this hall is a place where she works and earns rewards. Asking her to lie under a table through all of that is not a basic stay, it is an advanced settle with live distractions.
Build the under-table position at home first
Teach the geography of the behaviour where nothing else is competing. Put her mat under your kitchen or dining table, lure or guide her under, and reward her for lying down in that tucked position, which many dogs find odd at first because the space is low and enclosed. Practise while you sit at the table, stand up, walk around it and sit again, building to you eating a meal or working while she rests underneath. Add household distractions in stages, another person walking past, food being prepared, the doorbell recording, so the position itself becomes boring and well paid long before she is asked to hold it in a hall full of working dogs.
Work in short stints with real breaks at class
In class, treat her settle as a series of short reps rather than one long performance. Settle her under the table for a few minutes while one dog works, rewarding her quietly for lying down and for glancing at the working dog and then back at you. Then take a genuine break, a walk outside, a sniff and a drink, before her own exercise or the next watching stint. Choose your table position thoughtfully too: the end of a row, with more space from the working area, is a fairer starting point than the middle of the room.
Pay for watching calmly, not just for lying still
The behaviour you most need is not a frozen dog, it is a dog who can look at another dog working and choose to disengage. Reward that choice specifically: when she watches a recall or a retrieve and then turns her head back to you, mark it and pay her on her mat. Keep rewards calm and low, delivered down between her paws, so the paying itself does not become an exciting event that breaks the settle. If she fixates, whines or barks, increase her distance from the working area, turn her mat so her view is partly blocked by the table or your legs, and shorten the stint. Information from a failed stint tells you the exact step to practise at home that week.
Practise the class picture between classes
Recreate small pieces of the class environment in your weekly practice: a friend's dog doing a recall in the park while yours settles on her mat at a distance, training cues spoken by someone else, treats being delivered to another dog nearby. Move closer only when she can watch and turn back to you at the previous distance. This homework is what makes the next class easier, because the hall stops being the only place she ever meets that combination of triggers.
Track which class moments she can already handle
Progress at class is uneven, she may rest through stays and lead work long before she can watch retrieves or greeting exercises without joining in. Log the exercises she settled through, the distance that worked and where she needed a break, and keep those notes in KinsLe through Care Teams so anyone who takes her to class, or practises with her between sessions, works at the level she has actually reached.