TwoJeffs' hack is probably what you want. You have the option (each time a visit is generated) of allowing the normal visit, not allowing the visit but still getting the relationship bonus, or not allowing and not getting the bonus. Choose the flavor which matches what you have installed, under 'School Bus & Homework Hacks.'
http://www.simbology.com/smf/index.php?PHPSESSID=2762c07972a1dc2cceeee7d3789a6a38&topic=15.0Variant hack, geared by Monique for affecting bringing friend from work rather than from school:
http://www.modthesims2.com/download.php?t=135090.