My brother in law is accused of no reason

1. The correct charge, in my understanding would be S. 306 here. Anyway, I believe a bail application can be moved before the concerned magistrate, including requesting him to order the police to present the suicide note and all the records of the school discipline committee can be presented to back the case up for your brother in law.
2. I can not say as to how the Magistrate would react because Suicide cases as a result of administrative pressure in institutions are taken seriously. However, if the Supervisor only performed his duties in his official capacity, in all fairness and justification, I believe sufficient case can be made out for a bail and then later on it can be defended properly. In this case, it appears that there is some sort of back-channeling between the other accused persons in the suicide note and the investigation agency, which can also be mentioned in your application to the concerned Magistrate, who may then appropriately direct the Police to comply with his orders.
   
   I further suggest you to get some legal counsel or you can request for legal aid from the State as well to prepare a valid defense for the accused.

I fully agree with  the  answer of my learned friend Arjun Kohli.Nothing to say more.

First let me admitt I am not a professional. BUT i am giving my analysis hoping this may lead you to some new direction NOT from pure legal thinking.

You did not CLEARLY say what reasons he mentioned in his letter.

The entire case depends on this.YOU SAID:-

1. The boy committed suicide has several acquisition against his name by the school management.

here " several acquisitons against his name BY the school management"

? are those several   ? the schoold management

list ? are those several

NEXT what his share in managing the school with reference to his allegations.

.........................Does he have any discretionary powers

.........................using those discretionary powers has he taken such a decision without any reason.

WITH THE SAME APPROACH ANSWER THE FOLLOWING ALSO.:-

......That boy has earlier been found to be guilt of breaking rules and regulation of the School and the maters have earlier been taken to the cognizance of the school committee who have found that boy guilty. 

1. of the above:- Breaking rules and regulations....? 

..............? rules and regulations he broke 

.............? in questioning a student and taking the action against a student 

............................??? the role of your brother in law IN THE MANAGING COMMITTEE.

........................................Managing committee- means is it not a colletive decison of the committee OR to put it straight whether that management decision is taken by your brother in law IGNORING  the other committee members .

THUS IN MY OPTINION - IF YOUR BROTHER IN LAW CAN BE MADE SOLELY RESPONSIBLE - THEN ONLY HE CAN BE PUNISHED. if not those who framed and also implementing those rules are to be ?ed.

Hope this will give a non legal direction and make you think afresh.

IF NOT SIMPLEE IGNORE 

From the facts stated, the Police  are still investigating the case. All your brother-in-law can do is to cooperate with them. In the while, try to get competent legal assistance so that he can secure bail. The suicide by itsel is insuffiicent to imlicate him in the matter unless there are corroborating evidence.

Your brother in-law has been remanded to judicial custody (but not yet sentenced to jail as stated by you) wherein the trail of the case is yet to start after thourough investigation and filing of charge-sheet by police, as I can presume from the statements made by you, which are (prima facie) logically and factually incorrect, fabricated and baised.

Any how you have no alternative but to seek professional services of a local prudent lawyer practicing in criminal matters who shall advise/guide you and proceed, as required.

Therefore, there is no need to continue with this thread, close it.

"He is now given 10 days zail sentence. He is accused of IPC 302, The police however has not shown the suicide note in the Court. The others including the Principal of that school who is also accused in that suicide note has also been not brought to the purview of the Court."

The above facts are incoherent as observed by experts above. 

1. He cannot be sentenced on the same day (this happens in movies only). He may at the most be on Judicial custody deniying bail.
2. The given facts do not speak of murder so 302 is out of picture unless police is keen in spoiling the case and helping him.
3. the suicide note need not be proiduced now.  This vital evidence will beproduced by police with challan.

"But it is certainly not expected that a person should every time have a look towards each and every student and their activities"

apprently this can be no defenece.  His defenc elies in proving that whatever was the cause of the greivance of the deceased that was wholly and fully unjustified and that the action of the accused against himwere fully justified and legal.