"Does test preparation help improve student performance on the SAT and ACT? For students that have taken the test before and would like to boost their scores, coaching seems to help, but by a rather small amount. After controlling for group differences, the average coaching boost on the math section of the SAT is 14 to 15 points. The boost is smaller on the verbal section of the test, just 6 to 8 points. The combined effect of coaching on the SAT for the NELS sample is about 20 points. The effect of coaching is similar on comparable sections of the ACT. The average score increase on the ACT math section probably lies within the range of 0 to .4 points, while the coaching effect on the English section is about .3 to .6 points"
[1] http://nepc.colorado.edu/files/Briggs_Theeffectofadmissionst...