I don’t know if I’m the average student but I know what the effect was on me…a student in a relationship gets distracted much more easily. Especially in the early stages of the relationship, you never really stop thinking about that one special person, and so even when you’re “focused” half your brain is somewhere else.
A student may lose their motivation towards studies because the only thing that seems important is this person. So not only is it hard for them to focus, but they don’t particularly feel like trying either.
The student’s priorities and time management skills get jumbled as they start making time for this person in their life. They have less time to study because they spend more time out with their new significant other.
All of these factors can cause slips in a student’s grades, but as the relationship progresses and the student gets used to this extra person in their life, they can generally bring the grades back up. They sort their priorities out again, remember whatever it was that motivated them in the first place, and realize that it is not vastly important to the relationship to keep this person in their thoughts while trying to do Calculus.
That’s based on experience, but I’m sure it’s a bit different for each student and relationship.