Technically you could show up for class at 0800, notify them that you are going on mil leave and walk out at 0805 with your seniority clock ticking away..

If you are talking about monthly drill or AT, you will want to finish training first for your own benefit (including IOE), although you may be able to drill if you have gaps between class/sim/IOE.

If you are talking about long-term leave, yes it can legally be done. But at a small operation I suspect they would highlight you for special treatment upon your return...they might not flunk you (or they might) but it won't be fun.

If it's a large company, you might get lost in the shuffle with no adverse impact.

Now if this is a good faith situation, ie you got INVOL orders after you applied for the job then I would show up for class and explain the situation. Most companies would be fine with that.

But if you're taking voluntary leave just to doge first-year pay, they will not be too sympathetic.