The online classes are a really good system but, as I expected, doing it for more than one week, I lost interest and I cannot concentrate anymore.
「オンライン授業、すごくいいシステムだけど、１週間以上はさすがに飽きるし集中力が続かない。」は英語で「The online classes are a really good system but, as I expected, doing it for more than one week, I lost interest and I cannot concentrate anymore.」という表現が良いと思います。
オンライン授業ーOnline classes
すごくいいーVery good
一週間以上ーMore than one week
さすがにーAs expected・All the same
飽きるーLose interest・Get tired of
集中力ーConcentration
続かないーDon't continue
アメリカ合衆国 
