Google doesn’t have a sandbox to get out of. You never were in a sandbox. There is a common event that people often refer to as a “Sandbox Effect”. It’s not a penalty or even a bad thing.
Google ranks websites in its index with an algorithm that uses many different factors. One of these factors is the “freshness factor” (a.k.a. QDF) that will temporarily boost the ranking of newly added pages. After this “freshness factor” boost wears off, your web page will drop to its truly deserving rank.
Your new web pages were given a temporary boost (this is a good thing). Now go to work building traffic and backlinks so that you can earn a lasting higher ranking in Google’s index.