@davidd

Quote 1: Lewis shows that Corin is a spirited boy. His motive in running off is to defend Queen Susan's honour.

Quote 2: Corin should have had the discipline to ignore an idle comment from a boy in the street. Edmund tells us that Corin had disappeared over night. Corin should have known that all this time that he was missing would have been a problem. Arguably, he also should have known that getting into fights with kids in the street and the Calorman guard is not the best way to be an ambassador for your nation.

Though Corin could likely look after himself, Shasta did not know that when he hoped that he would be taken to Narnia and Archenland in Corin's place. There is still a failing here on Shasta's behalf.

Spirited boy or not, Corin did run away, and in doing so, let everyone down. In this story, I could see King Edmund's point of view, when he was friends with King Lune and when to mind anyone's child is a responsibility. And when even in VDT, talking to Eustace, he still is mindful of his own earlier behaviour in betraying his own family & Narnia to the White Witch in LWW. Edmund's then behaviour is attributed to the boarding school he went to, where, no doubt, the rules were strict. 

If his school was anything like the boarding school, I attended myself, the standard punishment for so-called "spirited boys who ran away" would be at least "6 of the best", administered by the School Superintendent or the Headmaster. I remember one boy in particular, one of a series, who did run away, name of "Peter Harvey", and what the Superintendent (who grew up in the same boarding school, himself) said about that disappointing boy compared to his well-behaved older sister Margaret, I think it was. I didn't really understand why the school was so strict until I was a parent, myself, & then forgave my Dad, for putting me into such a place when he couldn't work full-time & mind me, himself, & even in holidays he often left me with "minders" when he also had work to do. I can see why King Edmund would be furious, especially Susan, who, to be fair to her, would have taken her responsibilities for Corin even more seriously than she did as Edmund & Lucy's elder sister.

But getting back to Shasta, & given King Edmund's reaction on finding this Corin-lookalike, being used to being blamed & hit, I bet that he was scared stiff of what might happen to him, not only by then because of his own escape but also on Aravis' as well. 

Of course, his first impulse was to say that he was only poor Arsheesh the fisherman’s son and that the foreign lord must have mistaken him for someone else. But then, the very last thing he wanted to do in that crowded place was to start explaining who he was and what he was doing.

I'll bet. Shasta has very valid reasons for not wanting to say who he is, even though by now he knows he most certainly isn't "poor Arsheesh, the fisherman's son". We know how gruesome Calormen justice could be, not only from what Lasaraleen has to tell us, and now he not only has Arsheesh & Anradin (on account of Bree) after him, but Aravis's father looking for her - remember how Shasta sympathised with Aravis' maid, employed to spy on Aravis, herself, when her stepmother couldn't, & when Aravis' interests in horses & dogs were likely not considered "ladylike" by Calormene standards. What did @coracle say earlier in this thread about how Susan as a schoolgirl of 13 would be expected to behave?

@coracle However the actual life of a 13-year-old girl then was nothing like youth culture in the 21st century. Girls were treated as children, required to help around the home, do their schoolwork, and not talk to boys on the street.

Just like at my own boarding school, in fact, when the boys were in their own half & kept strictly away from girls, except when we were in class together. Unfortunately for them, maybe, the boys, not only the girls, had to not only learn to make their own beds - perfectly - army fashion & do 1/2 an hour a day each of housework & outside work, as well as schoolwork, but they were also expected, amongst other interests like drum-playing or sports, to do extra-curricular activities like running the school dairy and looking after the school's small herd of cattle. Even the senior girls on the school bus with the senior boys were not allowed to talk together, though brothers & sisters did try to keep in contact.

“There’s no time,” said Shasta in a frantic whisper. “I’m a Narnian, I believe; something Northern anyway. But I’ve been brought up all my life in Calormen. And I’m escaping: across the desert; with a talking Horse called Bree. And now, quick! How do I get away?”

Shasta, however much he enjoyed being coddled for a change, (thank you, Mr Tumnus, he might think), would realise the game's up once Corin appeared, so no wonder he was sort of half hoping that Corin wouldn't turn up, however selfish you may think he was. Apparently, Corin had learned to box but he still had a black eye, so of course, that would have to be explained, so he couldn't lie even if he wanted to. King Edmund didn't seem all that mollified when they all met up once again in Narnia, at Duffle, Rogin & Bricklethumb's abode.