BitLife Secondary School Explained
Secondary School in BitLife is just basically High School. This is where your Bitizen will spend four years of their life in the secondary or high school part of their education.

Secondary school is where your character will attend after elementary school, unless something gets in the way!
In the US the time at secondary school with be four years, but this will vary across different countries. Students will graduate anywhere between ages 13 and 19.
There are loads of things that you can accomplish in secondary school for various objectives.
For example as part of the Thank You, Next Challenge you need to 'Go out with 5+ Different People in Secondary School' This basically means that you need to date five or more people in secondary school.
Here's how to do that.
How to Go out with 5+ Different People in Secondary School
Begin friendships while in secondary school (high school), then eventually you'll get asked on a date. You'll get more dates if your Looks stat is higher. But you don't have to wait around to get asked to go on dates in High School in BitLife, you can also do it by selecting the Activities tab and choosing the Love menu, then Dating App.
Now just use the Dating App to find some dates in high school to fulfill the 'Go Out with 5+ Different People in Secondary School'
Why is is called Secondary School in BitLife?
Secondary is just another name for High School, so if you are used to the term High School, then basically Secondary School in the game is just the same thing, different countries have different lengths of time to attend high school.
In the US you'll spend 4 years there before either attending college or trying to get a job, so if you have a task to accomplish such as dating a certain number of people, make sure you get it done before graduating.