Teaching Science in English and using the old Teacher Directed method where the teacher is the only one getting a lot of practice speaking English while the students are just bystanders will not get much better results in improving students' English and it will have disastrous effects on their learning science.
In order for students to improve their English skills, they need to have opportunities to practice not only listening to a lot of foreign words, but to be able understand what they are hearing and lots and lots of opportunities to speak, to write and to read and should be as real as possible. When I say real, I mean it should be as connected and representative of things in their real life as possible.
I had someone once tell me that students could not learn a language without learning grammar. She was right, but nobody as an infant has had to learn the rules and verbs of their grammar in order to speak their language, they learned their grammar in the context of what they were learning.
This is where Cooperative Learning can really improve CLIL classes. In Cooperative Learning students are active participants of their learning and by doing so are getting many opportunities to speak in English to their peers, to their teacher. The teacher is a guide in the learning by Front-loading the new vocabulary. Using images and realia to make the new words as real as possible. The teacher also teaches students the skills they need to work effectively in groups. Research shows that Cooperative Learning increases retention which is what ELL need, instead of learning lots of new words that they forget as soon as they leave the classroom.