I used the word "suspected" and it was counted wrong. In English, in my area, you would use that word in the same way as "supposed" or "assumed." Example: I supposed that the student was a 4th grader, and I suspected that the student was a 4th grader, mean exactly the same thing; there need not be anything "suspicious" about that 4th grader.
What's interesting is that in another example provided by Duolingo:
Yes, I had already supposed it. → Sí, ya me lo había supuesto. Note the use of "me". But, in this example which is the same basic structure, they did not use se. This is what's really confusing sometimes about Duolingo.
Lo mostly means "it" or "him". It's a direct object pronoun and appears right in front of the conjugated verb (or attached to the end of certain verb forms).
Lo can also appear as neutral article when you give an article to an adjective that does not describe anything concrete. For instance: "I wish you the best" - "Deseo todo lo mejor".
Supuesto is from the verb suponer meaning to suppose or assume. Supuesto is also a noun meaning assumption. This DL sentence is using suponer in its' past participle conjugation which is similar in meaning to the English words ending in "ed" (walked, talked, assumed...) Había supuesto"(in this sentence) means "had assumed" and is part of the compound tenses in Spanish. The word "thought"" might better be translated using "pensar". This is an awkward sentence and in context might be more easily translated. Gotta enjoy the way DL makes us study!