A teacher's assistant (TA), is sometimes called an instructional assistant. These may either be volunteer or paid workers who help to assist teachers in accomplishing daily goals for each class. At minimum, the teacher's assistant usually has a high school diploma, but this is not always the case. Some assistants, especially in high school and junior high settings, are volunteers, and work to assist a teacher as an elective. They usually receive units for these jobs, which can last for a semester or a school year, and they tend to have demonstrated proficiency in the subject being taught, perhaps having taken the class the year prior and from the teacher they’d like to assist.

A volunteer teacher's assistant may also be a parent, or a college student. As funding has reduced the number of paid teaching assistant positions in particularly grammar schools, many parents have stepped in to help the teacher with all matters needed to run a class. These assistants may perform a variety of functions.

They may work with children in small groups, grade assignments, record grades, make copies or cut out papers for projects. They might also supervise a class for a short period of time if a teacher needs to make a trip to the office. Sometimes teachers will use a teacher's assistant to work with the students in most need of attention in the classroom, to give them extra time and attention that might be difficult to provide when a class is large or class abilities exhibit extraordinary range.

The paid teacher's assistant provides basically the same function as the volunteer parent or student. Unlike volunteers, a school employee must usually submit to background checks and fingerprinting to assure that the person has no background of criminal activity especially as related to children. This type of teacher's assistant may be called an instructional assistant to differentiate from the many volunteers who help teachers with day-to-day work and to signify that they are school employees. In the class setting, they may hold more responsibility and can supervise a class for longer breaks if a teacher needs to be absent for more than a few minutes.

Some instructional assistants are hired to work with a single student, usually one with significant behavioral issues or disabilities. This often allows these students to function in mainstream class environments, which if unassisted they could not accomplish. A teacher’s assistant may be especially accomplished in the areas of working with children with disabilities, and most often, special day classes may have the highest number of paid instructional assistants to meet needs of students with special requirements.

In college settings, many professors hire or supervise paid or unpaid teacher’s assistants. Sometimes these assistants are not only allowed to work with students, and grade and record grades, but they may also get to design instructional materials, and teach some portions of the class. Some majors require students to work as teacher’s assistants for at least a semester, and other times students merely want the experience of assisting professors and take a TA job in order to earn college credits. College students may also earn college credits for working as volunteer TAs in elementary or secondary school settings.