One of the problems when dancers try to build strength to improve développés is they target the wrong muscles. For developpe a la seconde, you need to work on hip flexion, external rotation and abduction. Leg strength alone isn't going to get your leg up there. You need to strengthen the hip flexors and train your body to hold your turnout throughout the extension (if you want specifics, you're working on the iliopsoas, rectus femoris and Sartorius). In extension a la seconde your muscle is in an extreme range of contraction, and so produces less force than at less extreme ranges, the only way to counter this is strength training!
So - squats, lunges, modified leg raises.
If you've been dancing for a long time, you're used to plieing every 2 minutes - so you're going to need weights for your squats to have any effect. Same with the lunges. Mix the lunges up - do side lunges, lunges with your back leg on a step or box, lunges onto wobble-boards - they'll all help. You can use any type of weights - dumbbells, barbells, kettlebells, a weighted medicine ball - whatever. Lift heavy.
It should feel like hard work, and you should be working at about 80% of your maximum. You're training for strength, so don't worry about bulking up, it isn't going to happen. Equally you're not looking at muscle endurance here, so don't go too light on the weights.
So... each set 3-5 times, 3 times a week, for 3 months. If it get's too easy increase your weights.
Standing Modified Leg Raises x8
Standing Modified Leg Raises with Resistance x8
Matt explains the modified leg raises:
Boom! High extension développés!