A teaching assistant or teacher's aide or education assistant is an individual who assists a teacher with instructional responsibilities. TAs include: graduate teaching assistants, who are graduate students; undergraduate teaching assistants, who are undergraduate students; secondary school TAs, who are either high school students or adults; and elementary school TAs, who are adults . By definition, TAs assist with classes, but many graduate students serve as the sole instructor for one or more classes each semester as a teaching fellow or graduate student instructor. Graduate and adult TAs generally have a fixed salary determined by each contract period ; however, undergraduates and high school students are sometimes unpaid and, in the US and other countries with the credit system, receive course credits in return for their assistance. Teaching assistants often help the main teacher by managing students with learning disabilities, such as ADHD, or even physical disabilities, such as blindness or deafness.