An important note is that the amount of experience awarded to all the participating drivers combined is more than the amount of experience you gain towards your B-Spec level.
Also, the experience is not shared equally, and appears to be related somewhat to how many laps each driver has completed, although I haven't done any testing on this. What I have observed is where the drivers complete a different number of 'stints', the driver who started, and has therefore done more 'stints' is awarded more xp than the driver who was last in the cycle. The cycle is from left to right across your selected drivers, starting from the driver you selected as the starting driver.
As you can run just one driver in any race where you are offered the opportunity to run more than one, using the above observations therefore it is possible to have a b-spec driver which has a higher level than your B-Spec level.