Most trade schools require that you have a high school diploma or general equivalency diploma (GED) before you can be admitted. This is particularly true if you wish to study a health profession (such as medical assistant or pharmacy technician) or a profession in which licensing is available only to high school graduates (such as firefighting). However, some schools will admit you to certain programs (such as cosmetology) if you have only two years of high school or if you are over 21 and pass an entrance exam.
If you need to be over a certain age to enter your chosen industry, most trade schools won't admit you until you have reached that age. For example, if you live in a state where you have to be at least 18 to be licensed as a radiology technician, your local schools probably will require that you be 18 to apply, or perhaps close enough to 18 that you'll turn 18 before finishing the program.
Most trade school programs do not have specific course prerequisites, but some do have them, especially for science-heavy programs such as nursing. Often, nursing programs will require that you have biology and chemistry courses on your high school transcript that you passed with a grade of C or higher before you can be admitted. (The more competitive the program, the higher your grades must be.) Similarly, in areas where certain programs are in high demand (dental hygiene is highly competitive in some regions) the science requirements might be more extensive and your grade point average (GPA) must be higher for you to be admitted.
If you are applying to health-related programs (such as surgical technician) or other training courses for fields that require strong judgment skills or quick thinking (such as criminal justice), you might need to provide references from teachers, employers or community leaders (such as your clergy person) attesting to your maturity, ability to interact well with people and intelligence.