I used to cheat in school and before I would cheat I would think “This is probably a sin” but I never looked into it deeply due to the fear of it being a sin. Is this a mortal sin or not?
So you didn’t know cheating/lying/ stealing (which is what doing that in school is) was a sin!? I think you should talk to your confessor. I also think you should be introspective enough to tell yourself the truth.
This ignorance can often be imputed to personal responsibility. This is the case when a man “takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin.” In such cases, the person is culpable for the evil he commits. http://www.vatican.va/archive/ccc_css/archive/catechism/p3s1c1a6.htm
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