Son in school fight, accidently hit teacher breaking up fight. Son charged with felony

Yup.  Classic law 101.  Your son intended on hitting Student, but hit teacher.  The intent was transferred when he hit the Teacher but the intent was still there.  He intended on committing a battery (the unlawful touching or harming of another); who he hit is of no consequence.

So, does your son have a lawyer? How old is he? Has he been charged as an adult?  Get a criminal defense attorney as soon as possible.  Does he qualify for a public defender? If not, try www.attorneypages.com, try the Indiana state bar or Indiana county bar associations in your area.  Also try legal aid. 

As to the specific laws:

IC 35-42-2
     Chapter 2. Battery and Related Offenses
IC 35-42-2-1

Battery
     Sec. 1. (a) A person who knowingly or intentionally touches another person in a rude, insolent, or angry manner commits battery, a Class B misdemeanor. However, the offense is:
        (1) a Class A misdemeanor if:
            (A) it results in bodily injury to any other person;
            (B) it is committed against a law enforcement officer or against a person summoned and directed by the officer while the officer is engaged in the execution of the officer's official duty;
            (C) it is committed against an employee of a penal facility or a juvenile detention facility (as defined in IC 31-9-2-71) while the employee is engaged in the execution of the employee's official duty;
            (D) it is committed against a firefighter (as defined in IC 9-18-34-1) while the firefighter is engaged in the execution of the firefighter's official duty;
            (E) it is committed against a community policing volunteer:
                (i) while the volunteer is performing the duties described in IC 35-41-1-4.7; or
                (ii) because the person is a community policing volunteer; or
            (F) it is committed against the state chemist or the state chemist's agent while the state chemist or the state chemist's agent is performing a duty under IC 15-16-5;
        (2) a Class D felony if it results in bodily injury to:
            (A) a law enforcement officer or a person summoned and directed by a law enforcement officer while the officer is engaged in the execution of the officer's official duty;
            (B) a person less than fourteen (14) years of age and is committed by a person at least eighteen (18) years of age;
            (C) a person of any age who has a mental or physical disability and is committed by a person having the care of the person with a mental or physical disability, whether the care is assumed voluntarily or because of a legal obligation;
            (D) the other person and the person who commits the battery was previously convicted of a battery in which the victim was the other person;
            (E) an endangered adult (as defined in IC 12-10-3-2);
            (F) an employee of the department of correction while the employee is engaged in the execution of the employee's official duty;
            (G) an employee of a school corporation while the employee is engaged in the execution of the employee's official duty;
            (H) a correctional professional while the correctional professional is engaged in the execution of the correctional

professional's official duty;
            (I) a person who is a health care provider (as defined in IC 16-18-2-163) while the health care provider is engaged in the execution of the health care provider's official duty;
            (J) an employee of a penal facility or a juvenile detention facility (as defined in IC 31-9-2-71) while the employee is engaged in the execution of the employee's official duty;
            (K) a firefighter (as defined in IC 9-18-34-1) while the firefighter is engaged in the execution of the firefighter's official duty;
            (L) a community policing volunteer:
                (i) while the volunteer is performing the duties described in IC 35-41-1-4.7; or
                (ii) because the person is a community policing volunteer; or
            (M) a family or household member (as defined in IC 35-41-1-10.6) if the person who committed the offense:
                (i) is at least eighteen (18) years of age; and
                (ii) committed the offense in the physical presence of a child less than sixteen (16) years of age, knowing that the child was present and might be able to see or hear the offense;

Here are the sentencing statutes: http://www.in.gov/legislative/ic/code/title35/ar50/ch2.html