Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Rough Collie, watch their skin and their confidence. Crusting or bald patches on the face, ear tips or legs can be an inherited skin and muscle condition, and the coat hides it well. A gentle collie who becomes snappy, or newly afraid, may be in pain. If they have been given any medicine and then wobble, drool or seem confused, treat it as an emergency: contact your vet now, and tell them they may carry MDR1. In an older collie, bumping into things or getting lost in the house is worth telling your vet about. Bloat (GDV, a twisted stomach) is most common in large, deep-chested dogs, so skip walks and hard play shortly before and after meals. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now.
He'll keep booking the children for grooming. If they find something under the fur, call your vet.