日本語の学校に行きたい I want to go to a Japanese school.

*「しかし」でも間違いではありませんが、「が」の方がより自然だと思います。

一人で勉強していても、十分上手ですよ。より上達するためにも、日本語学校に行けたらいいですね！

*To make this sentence more natural, it's better to use が. However, your sentence is not wrong! Make sense:)

You're good enough even though you study by yourself. In order to develop your Japanese more, it may be a good idea going Japanese language school. All the best!