There are several problems in your for loop - though your aggregation looks good.
Your result document "x" contains an array field "result". You want to iterate over the documents in that array. The for loop syntax you have won't set a to values in the array but rather to indexes in the array. Better and clearer would be to use forEach loop syntax:
Your update statement wasn't quite accurate either - you want to make sure you pull only the homework with the lowest score - what if the same student had a single exam which was also the same score as their lowest homework? Your syntax would pull all subdocuments which have the score equal to minscore!
That one is easy to fix (and I show you the syntax you need to use to only pull homework subdocuments), but remember that $pull will pull all values that match the query, and if there was only one homework then it's the minscore and it will get pulled. And if there were two homework scores but they were equal, they will both get pulled! Something to consider.
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