In GTA San Andreas there is adriving school where I can increase my car skill but how can I increase my bike skill?
There is a driving school at San Fierro near your Doherty garage and there is a bike school at Las Venturas. You can increase your skills there unless if you're still stuck at Los Santos.
If you mind changing some of the data in the game's files, go to the ar_stats.dat file inside the data folder of your main San Andreas directory and find the lines , which are these:
CODE STAT_INC_MOTORBIKE_SKILL 25 STAT_TIMELIMIT_MOTORBIKE_SKILL 180 STAT_INC_DRIVING_SKILL 20 STAT_TIMELIMIT_DRIVING_SKILL 300
Now change the 180 to 1 and the 300 to 1 too. Save and load up your game. Get in any car and drive around. You'll instantly notice your stats rapidly rising. The same will happen for your Motorbike skill when you rid a motorcycle.
This means that in normal gameplay time, 3 minutes of uninterrupted riding of a bike would increase the skill by 2.5%, while 5 minutes of uninterrupted driving of a car would raise the corresponding skill by 2%...