I am a junior in high school. I have had average GPAs throughout high school and this trimester (first trimester of Junior year) I got a 2.9 GPA. If I get a 4.0 or 3.9 for the next two trimesters of junior year and the rest of senior year, would it be more likely that colleges focus on my improvement later in high school or only the average GPA from earlier (BTW I go to an extremely difficult high school where regular classes here are considered as honor classes at other schools and I am taking two APs now because our school does not allow students to take APs until junior year).
An upward trend will help but might be a little too late depending upon what your GPA was prior to Junior year? what Freshman and Sophomore year GPA? Are there extenuating circumstances that have contributed to your low grades? Most colleges will only see up to Mid-year Senior transcripts so doing well the next 3 trimesters can help but will not make a huge impact on your GPA.
http://www.collegesimply.com/gpa-calculator/raise-gpa-from-2.8-to-3.0/
What miracle is going to happen? You’ve had average grades up till now and they will shoot up to a 3.9/4.0? It’s more important you figure out some good matches for your academic abilities.
If you know you can get the grades you want, you should try for them. At the worst you’ll probably get slightly more aid from your school, and at best you will open up new doors.