There was a scandal at my school many moons ago, when one of the teachers was found to be in a relationship with a 16 year old. Obviously teacher was very much fired and had to change career, but the relationship lasted, and they ended up married with a few kids and afaik are still together.
Personally I will always think that a teacher who hooks up with a teen is creepy as hell (power imbalance) but I suppose there is some sort of a grey area between 'perfectly professional' and 'outright predator' and it sounds to me like Russ is in it. His appeal for Lily is that he looks and sounds (fine if voice, accent, vocabulary) like a father figure, but he thinks like a teenager. All the fuss about what to take, what to wear, being overwhelmed by the house - is the stuff of a nervous 15 year old.
Incidentally, I wondered if Russ being so carried away by the size of the house was a jokey reference to that episode of More or Less, where they calculated that, going by the length of Nigel's scream, Lower Loxley is the height of York Minster.