Good story with minor flaws
First of all, let me say, that I liked your story.
Read the story now
I think it is very well written; your grammar and spelling are flawless as far as I could see and you really managed to pull me as a reader into your story.
I just have these minor grievances in the back of my head I can't get rid of. In particular with Sayre and Sam.
Sam first: he seems overly weak and sometimes stupid. He is a football player for his school but complains about the weight of Sayre's armor, which shouldn't weigh more than about 20 kg if Sayre wore a full plate armor which still wasn't invented at the time Sayre jumped out of his time. As for the stupid part: His rambling about Sayre's genes. Humanity as a whole has not changed all that much in the last 50000 years. yes we've grown a bit more intelligent, yes white people popped into existence, but that is about it. 600 years do nothing in terms of genetics, if the mutation that is causing the change does not bring about an overwhelming advantage for the mutated specimen. Since he is going to a fairly prestigious school, I assume this should have been tought to him in class already.
Now for Sayre: His armor is one of the things this nagging voice wouldn't shut up about. Correct me if I'm wrong, but I was under the impression, that Sayre wore a suit of chainmail armor under his plate armor. That would have been pointless, because plate armor was nigh impenetrable by most weapons at the time and if it were able to punch through plate armor it would most likely also penetrate chainmail.
Another thing was his offhand comment about tea in one of the nights he couldn't sleep. Tea wasn't brought to england until the 18th century and was extremely expensive even then; so Sayre wouldn't have known tea, even if it had been present at the time. The same goes for chocolate. He seemed to know it, or at least didn't ask something like "What is this chocolate you speak of", which would have been a natural reaction in my opinion.
Another thing about him is his build and the amount he eats. Either he should eat little, be lean but muscular in a wiry sense which would be sensible for a knight, or you want him to be this towering mountain of muscle and raw strength, which is also fine, but then he needs to eat more. He should be a veritable glutton, otherwise he wouldn't be able to stay healthy. His muscles would need ridiculous amounts of calories to fuel them and proteins to sustain them.
With all that said, please keep in mind they really are minor things I wanted to point out to you.