I was a high school senior last year... so i researched all i could about the college application process..... here's couple tips
1) Valedictorians, 1600 SAT, 4.0 GPA, National Merit applicants to top schools such as Harvard, Princeto, MIT, Duke, Stanford etc. are dimes a dozen...
Therefore, they tend to look beyond your academic grades/scores .... and put a heavy emphasis into factors such as E.C. (extracurricular), essay, and recommendations...
e.g. it is not good enough just to be the president of a club...but it is a different story if you are the president of a club that sends 1000lb of food and supplies to a needy 3rd world country every year ...
2) Slacking off during your 2nd semester in High School is OK to a certain extent ....
B's are ok ... 1 or 2 C's might be acceptable depending on the college....
D's and F's are definitely not good .... universities can and will rescind their admissions .....