First of all I can't believe that there was not even one decent junior dev among 400+. Last year I was in the same position as you but received far less applications (15-20). 3 of them was decent, one of them we employed and he turned out much better than on paper.
Main thing is to give them homework: not more than 2 hours project that they will solve at home and commit to private repo. Make it unique enough to avoid stackoverflow copy-pastes. Do not give them academic problems, ask them to create real world examples. For example: create CRUD that does something specific like file uploads with resize or similar; access unreliable API and do something with it; create calendar that allows multiple tasks per day (UI/UX); scrape data from the website and put it in to DB.
Those are just examples, but they are easy enough to do in under 2 hours and you can test results in under 2 minutes. Out of those 400+ applications, half of them will not even bother spending 2 hours on a task, then in a day or two you will restrict that remaining ones to about 50-60 just by checking results of the task. Rest are just personal interviews. Just remember to write a feedback email even to those unsuccessful applications.
Very interesting approach. I've heard of an approach similar to this one, although it was not limited to junior devs. You can quickly find out if a possible final candidate is appropriate, and a good fit for the company, by simply working with them for a day on a small project.
Main thing is to give them homework: not more than 2 hours project that they will solve at home and commit to private repo. Make it unique enough to avoid stackoverflow copy-pastes. Do not give them academic problems, ask them to create real world examples. For example: create CRUD that does something specific like file uploads with resize or similar; access unreliable API and do something with it; create calendar that allows multiple tasks per day (UI/UX); scrape data from the website and put it in to DB.
Those are just examples, but they are easy enough to do in under 2 hours and you can test results in under 2 minutes. Out of those 400+ applications, half of them will not even bother spending 2 hours on a task, then in a day or two you will restrict that remaining ones to about 50-60 just by checking results of the task. Rest are just personal interviews. Just remember to write a feedback email even to those unsuccessful applications.