Do colleges only look at Junior grades only?
Are junior year grades the most important? Yes, probably.
Do universities and colleges only look at Junior year grades: NO.
Most colleges look at freshman, sophomore, junior and first semester senior (or first quarter if applying ED/EA) year grades.
Sophomore and junior tend to be the most important. Within S and J year grades, “core” classes are looked at the most (sciences, english, history), and grades in classes like yearbook or photography are considered slightly less.
With that being said, this is not one-size-fits-all. Each university has its own method of evaluating grades.
Junior grade is most important because it may demonstrate upward trend. Also, it should have some rigor courses in it.
A student also want to do well in the Senior year since the final transcript will be sent to the school you chose to attend. There have been many seniors who played around that last year and then spend way too much time worried that their acceptances would be rescinded or that they would have to explain a severe downward trend in grades.