They're both fine. The only thing 'wrong' with your sentences is that you could only use them in the USA. The rest of the world - including Canada, Britain and Australia - uses Centigrade.

These seem perfectly understandable to me. The only thing that perhaps sounds "odd' -- not even "wrong" -- about the first is the use of "can". You might use that talking about climate and a continuing capability -- "Temperatures in the islands can go as high as the upper 70s during the summer" -- but not usually for the weather forecast for a specific time and place -- "could" works better in that case, because it's not certain. (If it was certain, you would use "will" as the modal.) (And to quibble, I might say "as high as 79" is a simpler way of putting "as high as the upper 70s", but it's true that people say things like that.)

English Ed is correct and so is Su.Ki Did someone tell you they're incorrect?

They are both fine. Why do you think they are wrong?