You can Log into each product in the Teachme series from the following URL.
https://teachme.jp/select-product
※Separate login URLs also exist for each individual product.
Additionally, if you are logged into any one of the Teachme series products, your login status will carry over to the other products as well.
Whether or not you can Log Out varies depending on the product.
You cannot Log Out from the Settings Center.
If you Log Out from Teachme Biz, you will also be logged out from the Settings Center.
Reference: Log in to the Teachme series selection screen
https://teachme.jp/8/manuals/51248656