If you're referring to the educational stage**:
* Undergraduate: This is the most direct opposite, referring to someone pursuing education before a graduate degree.
* Pre-college: This refers to the period of education before entering a university.
* Secondary: This term can be used to refer to high school or secondary education.
If you're referring to the level of education**:
* Beginner: This implies someone starting their education journey.
* Elementary: This refers to the early stages of education.
* Non-degree: This describes someone who is not pursuing a formal degree.
If you're referring to the status of being a student**:
* Alumnus/alumna: This refers to someone who has graduated from a particular educational institution.
* Professional: This describes someone working in a field after completing their education.
* Non-student: This simply means someone who is not currently enrolled in any educational program.
Ultimately, the best antonym for "postgraduate" depends on the specific context of your sentence.