most important 和 most importantly 都可以。

Most importantly, your time and effort at senior high school will open the door to your potential.

= Most important, your time and effort at senior high school will open the door to your potential.

但是，后面接有 of all短语时，则只能用 most important。如：

Most important of all, everyone has to stick by the rules. 最为重要的是，每个人都得遵守规章制度。

Most important of all, these activities have strengthened their ties with the working people. 最重要的是，这些活动加强了他们和劳动人民的联系。

注：类似的情况还有 more important 与 more importantly。如：

More important, you should not have consorted with the enemy. 更重要的是，你不该认敌为友。

More importantly [=It is more important that], the proposed law will have a harmful effect on funding for public schools. 更为重要的是，这一法案将对公立学校的拨款产生不利影响。

附：关于这个问题，夸克等编写的《英语语法大全》有详细说明，根据夸克的书：有人既反对使用most important，也反对使用most importantly。有些惯用法书籍提出用most important, 而另一些惯用法书籍则提出用most importantly。以下来自夸克书的截图：