There’s no right answer. Everybody is different. Some actors never even take drama classes. They just get a headshot and resume and start auditioning right away. That being said, I really recommend that if you’re serious about acting, you go to acting school. You need to get a complete training that includes not just drama classes but also movement, speech and voice classes, as well as other training like dance, stage combat, Shakespeare and acting for the camera. A good acting school will also help you get started in the business with things like casting director workshops and agent showcases. It can be very hard to get auditions at first if you don’t have any access to the professional world of acting.
If you took some drama classes in high school, you can certainly mention them on your resume until you get more professional training. You should also mention any on stage experience you have, even if it’s at your high school theater.
Even if you plan on going to acting school later, it’s great to take drama classes in high school. It will help you decide if you want to act as a career and your acting teacher can help you choose a good acting school or acting university based on your strengths and weaknesses as a performer.