There were definitely a few very bright kids in DC's class with high potential and their college goals were derailed by the choice of a more active social life. There is good advice above. If your child has a more impulsive nature and they are given a long leash, the next few years may not go as you hope.

Ask him/her what grade expectations are reasonable for this year. If they make a reasonable proposition, tie social activity to achieving those grades. If at the end of the first quarter, those aren't the grades they have earned, no socializing. (Or whatever limits on that you think are appropriate. One night/weekend or whatever but be specific up front and stick to it.) That way, they are meeting their own expectations (that you agree to) and not yours.

OP, responses may depend on what you consider to be "OK" grades.

I agree that gently bringing up college GPA requirements may be a motivating factor and it's probably not too soon for that.

Start college planning with DC. The reason they need to worry about grades is college admittance, since you said grades are 'OK' there is no danger of not graduating HS. College acceptance is more competitive than it used to be. If DC can see what it takes to be admitted to the program they want, perhaps that will be motivation enough. It worked for my kid.

I'd like to help get this school year off to a good start, without being overbearing/micromanaging. DC is rising 10th grader, and really loves their social life. School and grades, not so much. Grades are ok, but not putting in much effort (or caring). Maybe what I am looking for is how to turn back on the internal motivation that seemed to turn off last year. Any ideas?