How easy is it to get into Harvard with a perfect GPA at Colgate? Is retaking SATs allowed? Considering that my HS record isn’t stellar- but I am on my country’s 2018 IPhO team- what else do I need to do? Research? What else counts?
Virtually impossible. Harvard accepts about 12 transfer students a year - about half of whom are recruited athletes. They would want a compelling academic reason as to why you would want to transfer, and they will not believe that Colgate cannot provide similar opportunities as Harvard.
Additionally, if you applied to Harvard as a freshman and were rejected, it is most unlikely that there would be anything new in the intervening year that would make them reconsider.
What @skieurope said. In particular, unless you are an athlete you need a ‘compelling’ academic reason.
And Harvard can smell a prestige hound a mile away.
OP is still in HS. Apply to schools that fit you, and that you want to attend. Planning a transfer that far out is not a good plan for anyone involved.