As shown in the screenshot, I have some subclasses of Person, and I've created Instances for Educational Level such as First_Level, Second_Level etc..
I'm wondering how would I assign these to individuals I have created or would I create these subclasses within Data Properties and assign it to individuals that way?
Thought I would've been able to assign Instances to individuals, but the only options I have are Object and Data property assertions under individuals.
You cannot assign an instance to an individual - instance and individual are synonyms in this context. If you want to establish a relation between two instances (i.e., two individuals), you should use an object property.
That said, your taxonomy looks surprising to me. Why would a date of birth be a subclass of Person? A date is not a kind of person, in most domains. Perhaps those shouldn't be subclasses of Person, and this is why you're finding it difficult to model your assertions?