Thanks for query,
Dear this is quite early to masturbate.
Nothing to worry,this habit is not having any bed or side effect.
Problem with you is just psychological and try to forget it.
Do not see any sex movies or porno on computer which are main sources of get you sexually excited.
Concentrate your mind in study at this age.
Problem of not having concentration in school may be due to false fear about masturbation
You can take full breakfast before going to school so that if you do not take any thing is school will be ok.
Ok and bye,